Patent Publication Number: US-11034234-B2

Title: Systems and methods for fuel system pressure sensor rationalization

Description:
FIELD 
     The present description relates generally to methods and systems for determining an inherent offset for a fuel tank pressure transducer. 
     BACKGROUND/SUMMARY 
     Certain hybrid electric vehicles, such as plug in hybrid vehicles (PHEVs) for example, have sealed fuel systems. The reason for a sealed fuel system is because for such vehicles, engine run time may be limited, and thus opportunities for using engine manifold vacuum to purge a fuel vapor storage canister that captures and stores fuel vapors from the fuel system, may too be limited. As such vehicles may be driven in electric only mode, if the fuel system were not sealed, diurnal and running loss fuel vapors may overload the fuel vapor storage canister, which may result in undesired bleed emissions to atmosphere. 
     Such fuel systems may be sealed via a fuel tank isolation valve positioned in a conduit coupling a fuel tank to such a fuel vapor storage canister. In one example, to refuel a fuel tank that is otherwise sealed, a vehicle operator may depress a switch at a dash of the vehicle, and a controller may command the fuel tank unsealed. The controller may then monitor pressure in the fuel system, and when pressure in the fuel system is within a threshold of atmospheric pressure, then a fuel door may be commanded unlocked to enable refueling. 
     In another example, diagnostic routines may be conducted on a fuel system and/or evaporative emissions control systems of such vehicles, to confirm that the systems are not degraded, or in other words, to ensure that there are no sources of undesired evaporative emissions stemming from the fuel system and/or evaporative emissions system. For example, undesired evaporative emissions detection routines may be performed while the engine is running using engine intake manifold vacuum. In another example, undesired evaporative emissions detection routines may be performed when the vehicle is off using engine-off natural vacuum (EONV) generated due to a change in temperature and pressure within the fuel tank following engine shutdown and/or with vacuum supplemented from a vacuum pump. In some examples for sealed fuel systems, standing pressure or vacuum greater than predetermined thresholds in the fuel system during vehicle-off conditions may be indicative of a fuel system that is free from a source of undesired evaporative emissions. 
     Routines such as fuel tank depressurization and/or routines for checking a presence or an absence of undesired evaporative emissions rely on a functional fuel tank pressure transducer (FTPT) to measure the pressure or vacuum within the fuel system. As such, the rationality of the FTPT must be periodically tested and confirmed. The FTPT may be tested for offset, to determine if a baseline output of the FTPT is accurate. For example, if there is an inherent offset for the FTPT, then in some cases the fuel door may not open in response to a request to refuel, which may be frustrating and inconvenient to a vehicle operator or customer. In other examples, an inherent offset of the FTPT may lead to a determination that the fuel system is free from a presence of undesired evaporative emissions, when in fact it is not. 
     One example approach for an FTPT offset test is shown by Jentz et al. in US 2015/0075251. Therein, the fuel tank is vented to atmosphere for a lengthy vehicle-off soak. If the FTPT is functional, a value within a threshold of atmospheric pressure should be output following the vehicle-off soak. A deviation from atmospheric pressure may result in a diagnostic trouble code (DTC) being set at the controller, and/or may result in the FTPT output being adjusted to compensate for any offset. 
     However, the inventors herein have recognized potential issues with such systems. As discussed above, venting the fuel tank to atmosphere may result in fuel vapor trafficking to the fuel vapor canister, which may be undesirable for vehicles with limited engine run-time as opportunities for purging the fuel vapor canister to engine intake may be limited. If the engine must be forced on to purge the canister, the fuel efficiency of the vehicle may be reduced. 
     Furthermore, such an approach may not be possible for autonomous vehicles or other vehicles that participate in car-sharing models. Discussed herein, a car-sharing model includes a model of car rental where people rent vehicles for short periods of time. In some examples, a customer may pay for the use of such a vehicle by the hour, as a function of miles driven, etc. Such vehicles may accumulate much more mileage in a short period of time than vehicles that do not participate in car-sharing. Accordingly, long vehicle-off soak times may not regularly occur for such vehicles in which to determine FTPT offset. 
     In another example, U.S. Pat. No. 8,353,273 teaches coupling a fuel tank to a pump in order to generate a pressure signal in the fuel tank and at the position of the pump, and correlating fuel tank pressure with the pressure indicated at the pump. A fault signal may be generated responsive to the correlating. 
     However, the inventors herein have recognized potential issues with such a system. As one example, coupling the fuel tank to the pump may result in fuel vapors from the tank being routed to the fuel vapor canister, which is undesirable in vehicles with limited engine run-time, as discussed above. Furthermore, the use of an onboard pump adds costs and complexity to the engine system, and not all current and future vehicles are being designed with such a pump. A diagnostic that does not potentially load the canister and does not include the use of a pump is desirable. 
     In one example, the issues described above may be addressed by a method comprising waking a controller of a vehicle when it is expected that a pressure in a fuel system that is sealed from atmosphere is at atmospheric pressure, unsealing the fuel system, and indicating an offset of a pressure sensor used to monitor the pressure in the fuel system based on a pressure change as indicated via the pressure sensor after the fuel system is unsealed. In this way, the offset of the pressure sensor may be indicated without increasing a potential for release of undesired evaporative emissions to the environment. 
     The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings. 
     It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically shows an example vehicle propulsion system. 
         FIG. 2  schematically shows an example vehicle system with a fuel system and an evaporative emissions system. 
         FIG. 3  depicts an example method for determining potential issues associated with fuel tank depressurization in response to a request to refuel a fuel tank. 
         FIG. 4  depicts an example lookup table with several outcomes for diagnosing the fuel system according to the method depicted at  FIG. 3 . 
         FIG. 5A  depicts an example timeline for a first outcome of the lookup table depicted at  FIG. 4 . 
         FIG. 5B  depicts an example timeline for a second outcome of the lookup table depicted at  FIG. 4 . 
         FIG. 5C  depicts an example timeline for a third outcome of the lookup table depicted at  FIG. 4 . 
         FIG. 6  schematically shows a graphic depiction of a diurnal cycle. 
         FIG. 7  graphically depicts a transfer function used to convert fuel system pressure to a voltage for use with a comparator circuit depicted at  FIG. 8 . 
         FIG. 8  schematically shows an example comparator circuit for waking a controller of a vehicle based on fuel system pressure. 
         FIG. 9  depicts an example method for determining a fuel tank pressure transducer offset after the controller is woken via the comparator circuit of  FIG. 8 . 
         FIG. 10  depicts an example timeline for determining the fuel tank pressure transducer offset according to the method of  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION 
     The following description relates to systems and methods for diagnosing issues related to fuel system integrity. For example, fuel system integrity may relate to operational state of a fuel tank pressure transducer (FTPT) and/or to function of one or more valves configured to seal the fuel system. In some examples, fuel system integrity may relate to restrictions in one or more lines that couple the fuel system to atmosphere. The systems and methods discussed herein may be particularly useful for hybrid electric vehicles with limited engine run time, such as the hybrid vehicle depicted at  FIG. 1 .  FIG. 2  depicts a fuel system that is selectively coupled to a fuel vapor storage canister and to atmosphere via a fuel tank isolation valve (FTIV). A fuel tank pressure transducer is positioned in a line that couples the fuel system to the fuel vapor storage canister. In one example, the fuel tank pressure transducer may be relied upon for unlocking a refueling lock (e.g. fuel door) that provides access to a fuel filler neck for refueling a fuel tank of the fuel system. Specifically, in response to a request to refuel, the FTIV may be commanded open and the refueling lock may be unlocked responsive to the FTPT indicating that the fuel system is within a threshold range of atmospheric pressure. However, if the fuel system is not depressurizing and thus not allowing access to the fuel filler neck, then either the FTPT may be degraded (e.g. stuck in range or with an inherent offset), or there may be a restriction in the one or more lines that couple the fuel system to atmosphere (e.g. FTIV stuck closed and/or blockage in the one or more lines) that is preventing depressurization. Accordingly,  FIG. 3  depicts an example method for determining the source of the lack of depressurization in such situations. The example method depicted at  FIG. 3  may rely on both fuel system pressure measurements and fuel level measurements during refueling the fuel tank, and a diagnosis may be made based on a lookup table, such as the lookup table of  FIG. 4 . Such a lookup table includes three potential outcomes, each of which are described by the timelines of  FIGS. 5A-5C . 
     Thus, the method of  FIG. 3  relies on a refueling event, but it is herein recognized that for hybrid vehicles, such vehicles may go long periods of time without refueling, and thus another method for rationalizing the FTPT is presented that relies on diurnal cycle changes, such as the diurnal cycle depicted at  FIG. 6 . Such a method may include waking a controller of the vehicle at a particular time of a diurnal cycle when it is expected that the fuel system will be at or very near atmospheric pressure. In this way, the fuel system may be unsealed and FTPT offset determined, without undesirably loading the fuel vapor storage canister with fuel vapors, and/or without drawing in air into the fuel system. Such a method is depicted at  FIG. 9 , and relies on a transfer function as depicted at  FIG. 7  for latching an input voltage to a comparator circuit that is depicted at  FIG. 8 , such that the comparator circuit wakes the controller when the sealed fuel system is expected to be at or near atmospheric pressure in order to determine FTPT offset as via the method of  FIG. 9 . A timeline depicted how the method of  FIG. 9  is conducted, is depicted at  FIG. 10 . 
       FIG. 1  illustrates an example vehicle propulsion system  100 . Vehicle propulsion system  100  includes a fuel burning engine  110  and a motor  120 . As a non-limiting example, engine  110  comprises an internal combustion engine and motor  120  comprises an electric motor. Motor  120  may be configured to utilize or consume a different energy source than engine  110 . For example, engine  110  may consume a liquid fuel (e.g., gasoline) to produce an engine output while motor  120  may consume electrical energy to produce a motor output. As such, a vehicle with propulsion system  100  may be referred to as a hybrid electric vehicle (HEV). 
     Vehicle propulsion system  100  may utilize a variety of different operational modes depending on operating conditions encountered by the vehicle propulsion system. Some of these modes may enable engine  110  to be maintained in an off state (i.e., set to a deactivated state) where combustion of fuel at the engine is discontinued. For example, under select operating conditions, motor  120  may propel the vehicle via drive wheel  130  as indicated by arrow  122  while engine  110  is deactivated. 
     During other operating conditions, engine  110  may be set to a deactivated state (as described above) while motor  120  may be operated to charge energy storage device  150 . For example, motor  120  may receive wheel torque from drive wheel  130  as indicated by arrow  122  where the motor may convert the kinetic energy of the vehicle to electrical energy for storage at energy storage device  150  as indicated by arrow  124 . This operation may be referred to as regenerative braking of the vehicle. Thus, motor  120  can provide a generator function in some examples. However, in other examples, generator  160  may instead receive wheel torque from drive wheel  130 , where the generator may convert the kinetic energy of the vehicle to electrical energy for storage at energy storage device  150  as indicated by arrow  162 . 
     During still other operating conditions, engine  110  may be operated by combusting fuel received from fuel system  140  as indicated by arrow  142 . For example, engine  110  may be operated to propel the vehicle via drive wheel  130  as indicated by arrow  112  while motor  120  is deactivated. During other operating conditions, both engine  110  and motor  120  may each be operated to propel the vehicle via drive wheel  130  as indicated by arrows  112  and  122 , respectively. A configuration where both the engine and the motor may selectively propel the vehicle may be referred to as a parallel type vehicle propulsion system. Note that in some examples, motor  120  may propel the vehicle via a first set of drive wheels and engine  110  may propel the vehicle via a second set of drive wheels. 
     In other examples, vehicle propulsion system  100  may be configured as a series type vehicle propulsion system, whereby the engine does not directly propel the drive wheels. Rather, engine  110  may be operated to power motor  120 , which may in turn propel the vehicle via drive wheel  130  as indicated by arrow  122 . For example, during select operating conditions, engine  110  may drive generator  160  as indicated by arrow  116 , which may in turn supply electrical energy to one or more of motor  120  as indicated by arrow  114  or energy storage device  150  as indicated by arrow  162 . As another example, engine  110  may be operated to drive motor  120  which may in turn provide a generator function to convert the engine output to electrical energy, where the electrical energy may be stored at energy storage device  150  for later use by the motor. 
     Fuel system  140  may include one or more fuel storage tanks  144  for storing fuel on-board the vehicle. For example, fuel tank  144  may store one or more liquid fuels, including but not limited to: gasoline, diesel, and alcohol fuels. In some examples, the fuel may be stored on-board the vehicle as a blend of two or more different fuels. For example, fuel tank  144  may be configured to store a blend of gasoline and ethanol (e.g., E10, E85, etc.) or a blend of gasoline and methanol (e.g., M10, M85, etc.), whereby these fuels or fuel blends may be delivered to engine  110  as indicated by arrow  142 . Still other suitable fuels or fuel blends may be supplied to engine  110 , where they may be combusted at the engine to produce an engine output. The engine output may be utilized to propel the vehicle as indicated by arrow  112  or to recharge energy storage device  150  via motor  120  or generator  160 . 
     In some examples, energy storage device  150  may be configured to store electrical energy that may be supplied to other electrical loads residing on-board the vehicle (other than the motor), including cabin heating and air conditioning, engine starting, headlights, cabin audio and video systems, etc. As a non-limiting example, energy storage device  150  may include one or more batteries and/or capacitors. 
     Control system  190  may communicate with one or more of engine  110 , motor  120 , fuel system  140 , energy storage device  150 , and generator  160 . Control system  190  may receive sensory feedback information from one or more of engine  110 , motor  120 , fuel system  140 , energy storage device  150 , and generator  160 . Further, control system  190  may send control signals to one or more of engine  110 , motor  120 , fuel system  140 , energy storage device  150 , and generator  160  responsive to this sensory feedback. Control system  190  may receive an indication of an operator requested output of the vehicle propulsion system from a vehicle operator  102 . For example, control system  190  may receive sensory feedback from pedal position sensor  194  which communicates with pedal  192 . Pedal  192  may refer schematically to a brake pedal and/or an accelerator pedal. Furthermore, in some examples control system  190  may be in communication with a remote engine start receiver  195  (or transceiver) that receives wireless signals  106  from a key fob  104  having a remote start button  105 . In other examples (not shown), a remote engine start may be initiated via a cellular telephone, or smartphone based system where a user&#39;s cellular telephone sends data to a server and the server communicates with the vehicle to start the engine. 
     Energy storage device  150  may periodically receive electrical energy from a power source  180  residing external to the vehicle (e.g., not part of the vehicle) as indicated by arrow  184 . As a non-limiting example, vehicle propulsion system  100  may be configured as a plug-in hybrid electric vehicle (HEV), whereby electrical energy may be supplied to energy storage device  150  from power source  180  via an electrical energy transmission cable  182 . During a recharging operation of energy storage device  150  from power source  180 , electrical transmission cable  182  may electrically couple energy storage device  150  and power source  180 . While the vehicle propulsion system is operated to propel the vehicle, electrical transmission cable  182  may disconnected between power source  180  and energy storage device  150 . Control system  190  may identify and/or control the amount of electrical energy stored at the energy storage device, which may be referred to as the state of charge (SOC). 
     In other examples, electrical transmission cable  182  may be omitted, where electrical energy may be received wirelessly at energy storage device  150  from power source  180 . For example, energy storage device  150  may receive electrical energy from power source  180  via one or more of electromagnetic induction, radio waves, and electromagnetic resonance. As such, it should be appreciated that any suitable approach may be used for recharging energy storage device  150  from a power source that does not comprise part of the vehicle. In this way, motor  120  may propel the vehicle by utilizing an energy source other than the fuel utilized by engine  110 . 
     Fuel system  140  may periodically receive fuel from a fuel source residing external to the vehicle. As a non-limiting example, vehicle propulsion system  100  may be refueled by receiving fuel via a fuel dispensing device  170  as indicated by arrow  172 . In some examples, fuel tank  144  may be configured to store the fuel received from fuel dispensing device  170  until it is supplied to engine  110  for combustion. In some examples, control system  190  may receive an indication of the level of fuel stored at fuel tank  144  via a fuel level sensor. The level of fuel stored at fuel tank  144  (e.g., as identified by the fuel level sensor) may be communicated to the vehicle operator, for example, via a fuel gauge or indication in a vehicle instrument panel  196 . 
     The vehicle propulsion system  100  may also include an ambient temperature/humidity sensor  198 , and a roll stability control sensor, such as a lateral and/or longitudinal and/or yaw rate sensor(s)  199 . The vehicle instrument panel  196  may include indicator light(s) and/or a text-based display in which messages are displayed to an operator. The vehicle instrument panel  196  may also include various input portions for receiving an operator input, such as buttons, touch screens, voice input/recognition, etc. For example, the vehicle instrument panel  196  may include a refueling button  197  which may be manually actuated or pressed by a vehicle operator to initiate refueling. For example, in response to the vehicle operator actuating refueling button  197 , a fuel tank in the vehicle may be depressurized so that refueling may be performed. In some examples, a refueling lock (e.g. fuel door, etc.) may be manually opened via a vehicle operator depressing a manual refueling lock button  191 . Such a manual refueling lock button may be positioned in a trunk of the vehicle, for example. 
     Control system  190  may be communicatively coupled to other vehicles or infrastructures using appropriate communications technology, as is known in the art. For example, control system  190  may be coupled to other vehicles or infrastructures via a wireless network  131 , which may comprise Wi-Fi, Bluetooth, a type of cellular service, a wireless data transfer protocol, and so on. Control system  190  may broadcast (and receive) information regarding vehicle data, vehicle diagnostics, traffic conditions, vehicle location information, vehicle operating procedures, etc., via vehicle-to-vehicle (V2V), vehicle-to-infrastructure-to-vehicle (V2I2V), and/or vehicle-to-infrastructure (V2I or V2X) technology. The communication and the information exchanged between vehicles can be either direct between vehicles, or can be multi-hop. In some examples, longer range communications (e.g. WiMax) may be used in place of, or in conjunction with, V2V, or V2I2V, to extend the coverage area by a few miles. In still other examples, vehicle control system  190  may be communicatively coupled to other vehicles or infrastructures via a wireless network  131  and the internet (e.g. cloud), as is commonly known in the art. In some examples, control system may be coupled to other vehicles or infrastructures via wireless network  131 , in order to retrieve information that may be applicable to route-learning, as will be discussed in detail below. 
     Vehicle system  100  may also include an on-board navigation system  132  (for example, a Global Positioning System) that an operator of the vehicle may interact with. The navigation system  132  may include one or more location sensors for assisting in estimating vehicle speed, vehicle altitude, vehicle position/location, etc. This information may be used to infer engine operating parameters, such as local barometric pressure. As discussed above, control system  190  may further be configured to receive information via the internet or other communication networks. Information received from the GPS may be cross-referenced to information available via the internet to determine local weather conditions, local vehicle regulations, etc. In one example, information received from the GPS may be utilized in conjunction with route learning methodology, such that routes commonly traveled by a vehicle may be learned by the vehicle control system  190 . In some examples, other sensors, such as lasers, radar, sonar, acoustic sensors, etc., (e.g.  133 ) may be additionally or alternatively utilized in conjunction with the onboard navigation system to conduct route learning of commonly traveled routes by the vehicle. 
       FIG. 2  shows a schematic depiction of a vehicle system  206 . It may be understood that vehicle system  206  may comprise the same vehicle system as vehicle system  100  depicted at  FIG. 1 . The vehicle system  206  includes an engine system  208  coupled to an emissions control system (evaporative emissions system)  251  and a fuel system  218 . It may be understood that fuel system  218  may comprise the same fuel system as fuel system  140  depicted at  FIG. 1 . Emission control system  251  includes a fuel vapor storage container or canister  222  which may be used to capture and store fuel vapors. In some examples, vehicle system  206  may be a hybrid electric vehicle system. However, it may be understood that the description herein may refer to a non-hybrid vehicle, for example a vehicle only-equipped with an engine and not an onboard energy storage device, without departing from the scope of the present disclosure. 
     The engine system  208  may include an engine  110  having a plurality of cylinders  230 . The engine  110  includes an engine air intake  223  and an engine exhaust  225 . The engine air intake  223  includes a throttle  262  in fluidic communication with engine intake manifold  244  via an intake passage  242 . Further, engine air intake  223  may include an air box and filter (not shown) positioned upstream of throttle  262 . The engine exhaust system  225  includes an exhaust manifold  248  leading to an exhaust passage  235  that routes exhaust gas to the atmosphere. The engine exhaust system  225  may include one or more exhaust catalyst  270 , which may be mounted in a close-coupled position in the exhaust. One or more emission control devices may include a three-way catalyst, lean NOx trap, diesel particulate filter, oxidation catalyst, etc. It will be appreciated that other components may be included in the engine such as a variety of valves and sensors. For example, a barometric pressure sensor  213  may be included in the engine intake. In one example, barometric pressure sensor  213  may be a manifold air pressure (MAP) sensor and may be coupled to the engine intake downstream of throttle  262 . Barometric pressure sensor  213  may rely on part throttle or full or wide open throttle conditions, e.g., when an opening amount of throttle  262  is greater than a threshold, in order accurately determine BP. 
     Fuel system  218  may include a fuel tank  220  coupled to a fuel pump system  221 . It may be understood that fuel tank  220  may comprise the same fuel tank as fuel tank  144  depicted above at  FIG. 1 . The fuel pump system  221  may include one or more pumps for pressurizing fuel delivered to the injectors of engine  110 , such as the example injector  266  shown. While only a single injector  266  is shown, additional injectors are provided for each cylinder. It will be appreciated that fuel system  218  may be a return-less fuel system, a return fuel system, or various other types of fuel system. Fuel tank  220  may hold a plurality of fuel blends, including fuel with a range of alcohol concentrations, such as various gasoline-ethanol blends, including E10, E85, gasoline, etc., and combinations thereof. A fuel level sensor  234  located in fuel tank  220  may provide an indication of the fuel level (“Fuel Level Input”) to controller  212 . As depicted, fuel level sensor  234  may comprise a float connected to a variable resistor. Alternatively, other types of fuel level sensors may be used. 
     Vapors generated in fuel system  218  may be routed to an evaporative emissions control system  251  which includes a fuel vapor canister  222  via vapor recovery line  231 , before being purged to the engine air intake  223 . Vapor recovery line  231  may be coupled to fuel tank  220  via one or more conduits and may include one or more valves for isolating the fuel tank during certain conditions. For example, vapor recovery line  231  may be coupled to fuel tank  220  via one or more or a combination of conduits  271 ,  273 , and  275 . 
     Further, in some examples, one or more fuel tank vent valves may be positioned in conduits  271 ,  273 , or  275 . Among other functions, fuel tank vent valves may allow a fuel vapor canister of the emissions control system to be maintained at a low pressure or vacuum without increasing the fuel evaporation rate from the tank (which would otherwise occur if the fuel tank pressure were lowered). For example, conduit  271  may include a grade vent valve (GVV)  287 , conduit  273  may include a fill limit venting valve (FLVV)  285 , and conduit  275  may include a grade vent valve (GVV)  283 . Further, in some examples, recovery line  231  may be coupled to a fuel filler system  219 . In some examples, fuel filler system may include a fuel cap  205  for sealing off the fuel filler system from the atmosphere. Refueling system  219  is coupled to fuel tank  220  via a fuel filler pipe or neck  211 . 
     Further, refueling system  219  may include refueling lock  245 . In some examples, refueling lock  245  may be a fuel cap locking mechanism. The fuel cap locking mechanism may be configured to automatically lock the fuel cap in a closed position so that the fuel cap cannot be opened. For example, the fuel cap  205  may remain locked via refueling lock  245  while pressure or vacuum in the fuel tank is greater than a threshold. In response to a refuel request, e.g., a vehicle operator initiated request, the fuel tank may be depressurized and the fuel cap unlocked after the pressure or vacuum in the fuel tank falls below a threshold. A fuel cap locking mechanism may be a latch or clutch, which, when engaged, prevents the removal of the fuel cap. The latch or clutch may be electrically locked, for example, by a solenoid, or may be mechanically locked, for example, by a pressure diaphragm. 
     In some examples, refueling lock  245  may be a filler pipe valve located at a mouth of fuel filler pipe  211 . In such examples, refueling lock  245  may not prevent the removal of fuel cap  205 . Rather, refueling lock  245  may prevent the insertion of a refueling pump into fuel filler pipe  211 . The filler pipe valve may be electrically locked, for example by a solenoid, or mechanically locked, for example by a pressure diaphragm. 
     In some examples, refueling lock  245  may be a refueling door lock, such as a latch or a clutch which locks a refueling door located in a body panel of the vehicle. The refueling door lock may be electrically locked, for example by a solenoid, or mechanically locked, for example by a pressure diaphragm. 
     In examples where refueling lock  245  is locked using an electrical mechanism, refueling lock  245  may be unlocked by commands from controller  212 , for example, when a fuel tank pressure decreases below a pressure threshold. In examples where refueling lock  245  is locked using a mechanical mechanism, refueling lock  245  may be unlocked via a pressure gradient, for example, when a fuel tank pressure decreases to atmospheric pressure. 
     Emissions control system  251  may include one or more emissions control devices, such as one or more fuel vapor canisters  222  filled with an appropriate adsorbent  286   b , the canisters are configured to temporarily trap fuel vapors (including vaporized hydrocarbons) during fuel tank refilling operations and “running loss” (that is, fuel vaporized during vehicle operation). In one example, the adsorbent  286   b  used is activated charcoal. Emissions control system  251  may further include a canister ventilation path or vent line  227  which may route gases out of the canister  222  to the atmosphere when storing, or trapping, fuel vapors from fuel system  218 . 
     Canister  222  may include a buffer  222   a  (or buffer region), each of the canister and the buffer comprising the adsorbent. As shown, the volume of buffer  222   a  may be smaller than (e.g., a fraction of) the volume of canister  222 . The adsorbent  286   a  in the buffer  222   a  may be same as, or different from, the adsorbent in the canister (e.g., both may include charcoal). Buffer  222   a  may be positioned within canister  222  such that during canister loading, fuel tank vapors are first adsorbed within the buffer, and then when the buffer is saturated, further fuel tank vapors are adsorbed in the canister. In comparison, during canister purging, fuel vapors are first desorbed from the canister (e.g., to a threshold amount) before being desorbed from the buffer. In other words, loading and unloading of the buffer is not linear with the loading and unloading of the canister. As such, the effect of the canister buffer is to dampen any fuel vapor spikes flowing from the fuel tank to the canister, thereby reducing the possibility of any fuel vapor spikes going to the engine. One or more temperature sensors  232  may be coupled to and/or within canister  222 . As fuel vapor is adsorbed by the adsorbent in the canister, heat is generated (heat of adsorption). Likewise, as fuel vapor is desorbed by the adsorbent in the canister, heat is consumed. In this way, the adsorption and desorption of fuel vapor by the canister may be monitored and estimated based on temperature changes within the canister. 
     Vent line  227  may also allow fresh air to be drawn into canister  222  when purging stored fuel vapors from fuel system  218  to engine intake  223  via purge line  228  and purge valve  261 . For example, purge valve  261  may be normally closed but may be opened during certain conditions so that vacuum from engine intake manifold  244  is provided to the fuel vapor canister for purging. In some examples, vent line  227  may include an air filter  259  disposed therein upstream of a canister  222 . 
     In some examples, the flow of air and vapors between canister  222  and the atmosphere may be regulated by a canister vent valve  297  coupled within vent line  227 . When included, the canister vent valve  297  may be a normally open valve, so that fuel tank isolation valve  252  (FTIV) may control venting of fuel tank  220  with the atmosphere. FTIV  252  may be positioned between the fuel tank and the fuel vapor canister  222  within conduit  278 . FTIV  252  may be a normally closed valve, that when opened, allows for the venting of fuel vapors from fuel tank  220  to fuel vapor canister  222 . Fuel vapors may then be vented to atmosphere, or purged to engine intake system  223  via canister purge valve  261 . 
     Fuel system  218  may be operated by controller  212  in a plurality of modes by selective adjustment of the various valves and solenoids. It may be understood that control system  214  may comprise the same control system as control system  190  depicted above at  FIG. 1 . For example, the fuel system may be operated in a fuel vapor storage mode (e.g., during a fuel tank refueling operation and with the engine not combusting air and fuel), wherein the controller  212  may open isolation valve  252  (when included) while closing canister purge valve (CPV)  261  to direct refueling vapors into canister  222  while preventing fuel vapors from being directed into the intake manifold. 
     As another example, the fuel system may be operated in a refueling mode (e.g., when fuel tank refueling is requested by a vehicle operator), wherein the controller  212  may open isolation valve  252  (when included), while maintaining canister purge valve  261  closed, to depressurize the fuel tank before allowing enabling fuel to be added therein. As such, isolation valve  252  (when included) may be kept open during the refueling operation to allow refueling vapors to be stored in the canister. After refueling is completed, the isolation valve may be closed. 
     As yet another example, the fuel system may be operated in a canister purging mode (e.g., after an emission control device light-off temperature has been attained and with the engine combusting air and fuel), wherein the controller  212  may open canister purge valve  261  while closing isolation valve  252  (when included). Herein, the vacuum generated by the intake manifold of the operating engine may be used to draw fresh air through vent  227  and through fuel vapor canister  222  to purge the stored fuel vapors into intake manifold  244 . In this mode, the purged fuel vapors from the canister are combusted in the engine. The purging may be continued until the stored fuel vapor amount in the canister is below a threshold. 
     Controller  212  may comprise a portion of a control system  214 . In some examples, control system  214  may be the same as control system  190 , illustrated in  FIG. 1 . Control system  214  is shown receiving information from a plurality of sensors  216  (various examples of which are described herein) and sending control signals to a plurality of actuators  281  (various examples of which are described herein). As one example, sensors  216  may include exhaust gas sensor  237  located upstream of the emission control device  270 , temperature sensor  233 , pressure sensor  291 , pressure sensor  282 , and canister temperature sensor  232 . Other sensors such as pressure, temperature, air/fuel ratio, and composition sensors may be coupled to various locations in the vehicle system  206 . As another example, the actuators may include throttle  262 , fuel tank isolation valve  252 , canister purge valve  261 , and canister vent valve  297 . The control system  214  may include a controller  212 . The controller may receive input data from the various sensors, process the input data, and trigger the actuators in response to the processed input data based on instruction or code programmed therein corresponding to one or more routines. Example control routines are described herein with regard to  FIG. 3  and  FIG. 9 . 
     In some examples, the controller may be placed in a reduced power mode or sleep mode, wherein the controller maintains essential functions only, and operates with a lower battery consumption than in a corresponding awake mode. For example, the controller may be placed in a sleep mode following a vehicle-off event in order to perform a diagnostic routine at a duration after the vehicle-off event. The controller may have a wake input that allows the controller to be returned to an awake mode based on an input received from one or more sensors. 
     Undesired evaporative emissions detection routines may be intermittently performed by controller  212  on fuel system  218  and/or evaporative emissions system  251  to confirm that undesired evaporative emissions are not present in the fuel system and/or evaporative emissions system. As such, evaporative emissions detection routines may be performed while the engine is off (engine-off test) using engine-off natural vacuum (EONV) generated due to a change in temperature and pressure at the fuel tank following engine shutdown and/or with vacuum supplemented from a vacuum pump. Alternatively, evaporative emissions detection routines may be performed while the engine is running by operating a vacuum pump (where included) and/or using engine intake manifold vacuum. In some configurations, a canister vent valve (CVV)  297  may be coupled within vent line  227 . CVV  297  may function to adjust a flow of air and vapors between canister  222  and the atmosphere. The CVV may also be used for diagnostic routines. When included, the CVV may be opened during fuel vapor storing operations (for example, during fuel tank refueling and while the engine is not running) so that air, stripped of fuel vapor after having passed through the canister, can be pushed out to the atmosphere. Likewise, during purging operations (for example, during canister regeneration and while the engine is running), the CVV may be opened to allow a flow of fresh air to strip the fuel vapors stored in the canister. In some examples, CVV  297  may be a solenoid valve wherein opening or closing of the valve is performed via actuation of a canister vent solenoid. In particular, the canister vent valve may be an open that is closed upon actuation of the canister vent solenoid. In some examples, CVV  297  may be configured as a latchable solenoid valve. In other words, when the valve is placed in a closed configuration, it latches closed without requiring additional current or voltage. For example, the valve may be closed with a 100 ms pulse, and then opened at a later time point with another 100 ms pulse. In this way, the amount of battery power required to maintain the CVV closed is reduced. In particular, the CVV may be closed while the vehicle is off, thus maintaining battery power while maintaining the fuel emissions control system sealed from atmosphere. 
     Conducting an EONV test may include four “phases”. The first phase may comprise an initial vent phase. This initial vent phase is conducted to vent any vapors from a fuel slosh event from a hard stop just prior to key off. The initial vent phase may comprise 30-60 seconds, for example. The next phase of the EONV test may constitute a pressure phase. In this phase, the fuel system and evaporative emissions systems are sealed from atmosphere, and a pressure build is monitored over time. If the pressure in the fuel system and evaporative emissions system reaches a positive pressure threshold, an absence of undesired evaporative emissions may be indicated. However, if the pressure build stalls (e.g. plateaus), then the pressure in the fuel system and evaporative emissions system may be relieved, during what is referred to as the vent phase. After pressure in the fuel tank and evaporative emissions system is relieved, a vacuum phase comes next. The vacuum phase may include re-sealing the fuel system and evaporative emissions system, and monitoring a vacuum build over time. If vacuum builds to a negative pressure threshold within a predetermined duration (e.g. 45 minutes since the start of the EONV test), then an absence of undesired evaporative emissions may be indicated. 
     In some examples where the vehicle includes a sealed fuel tank, standing pressure in the sealed fuel tank may be indicative of an absence of undesired evaporative emissions. For example, if pressure greater than (e.g. more positive or more negative than) a predetermined absolute pressure threshold is indicated in the fuel tank, then the absence of undesired evaporative emissions may be indicated. However, if there is an inherent offset associated with the FTPT, then there may be circumstances where the presence of undesired evaporative emissions in the fuel system may be incorrectly diagnosed as an absence of undesired evaporative emissions. 
     Furthermore, as discussed above, an inherent offset associated with the FTPT may result in a fuel door or other refueling lock associated with accessing a fuel filler neck (e.g.  211 ) unable to be opened, due to an indication of pressure in the fuel system, where in fact the fuel system may be at atmospheric pressure. However, there may be other reasons for the fuel system not depressurizing, such as a stuck closed FTIV, for example. It is herein recognized that under such a circumstance where the fuel lock does not open due to an indication that the fuel system is not sufficiently depressurized, if the fuel lock is unlocked manually, the ensuing refueling event may provide an opportunity to diagnose the reason as to why the fuel lock did not open until it was manually opened. For example, the FTPT may have an inherent offset or may be stuck in range, or the FTIV may be stuck closed. The method discussed below at  FIG. 3  may enable a determination as to which may be the case for the particular situation. 
     Turning now to  FIG. 3 , an example method  300  for diagnosing a vehicle fuel system during a refueling event, is shown. Specifically, method  300  may be used to diagnose whether a standing pressure in the vehicle fuel system is due to an FTPT offset or otherwise degraded FTPT, or to a stuck closed FTIV and/or restriction or blockage in one or more lines coupling the fuel system to atmosphere. Method  300  may be used during a refueling event where it is indicated that the fuel filler neck has been accessed manually, for example via a button (e.g.  191 ) in a trunk of the vehicle. 
     Method  300  will be described with reference to the systems described herein and shown in  FIGS. 1-2 , though it should be understood that similar methods may be applied to other systems without departing from the scope of this disclosure. Method  300  may be carried out by a controller, such as controller  212  in  FIG. 2 , and may be stored at the controller as executable instructions in non-transitory memory. Instructions for carrying out method  300  and the rest of the methods included herein may be executed by the controller based on instructions stored on a memory of the controller and in conjunction with signals received from sensors of the engine system, such as the sensors described above with reference to  FIGS. 1-2 . The controller may employ actuators such as FTIV (e.g.  252 ), etc., to alter states of devices in the physical world according to the method depicted below. 
     Method  300  begins at  305 , and includes estimating and/or measuring vehicle operating conditions. Operating conditions may be estimated, measured, and/or inferred, and may include one or more vehicle conditions, such as vehicle speed, vehicle location, etc., various engine conditions, such as engine status, engine load, engine speed, A/F ratio, manifold air pressure, etc., various fuel system conditions, such as fuel level, fuel type, fuel temperature, etc., various evaporative emissions system conditions, such as fuel vapor canister load, fuel tank pressure, etc., as well as various ambient conditions, such as ambient temperature, humidity, barometric pressure, etc. 
     Proceeding to  310 , method  300  may include indicating whether refueling is requested by a vehicle operator. For example, refueling may be indicated to be requested if a refueling button (e.g.  197 ) associated with the vehicle instrument panel (e.g.  196 ) has been depressed. It may be understood that such a request may not unlock a refueling lock (e.g.  145 ), but may only signal the request to refuel, which may then be enabled in response to pressure in the fuel system becoming within a threshold (e.g. +/−5 InH2O) of atmospheric pressure. 
     If refueling is not requested, method  300  may proceed to  315 . At  315 , method  300  may include maintaining current vehicle operating parameters. For example, if the vehicle is operating via an electric-only mode, then such a mode of operation may be maintained. If the vehicle is operating via the engine, or some combination of the engine and the electrical mode of operation, then such operation may be maintained. Method  300  may then end. 
     Returning to  310 , in response to refueling being indicated to be requested, method  300  may proceed to  320 . At  320 , method  300  may include commanding open the FTIV. While not explicitly illustrated, commanding open the FTIV at  320  may further include commanding open or maintaining open the CVV (e.g.  297 ), such that the fuel system is fluidically coupled to atmosphere (provided the FTIV is functioning as desired). 
     It may be understood that with the fuel system coupled to atmosphere, it may be expected that pressure in the fuel system may decrease to within the threshold of atmospheric pressure. However, in a case where the FTPT (e.g.  291 ) has an inherent offset, or in a case where the FTIV did not open as expected (or where there are other restrictions in the lines coupling the fuel system to atmosphere), then pressure may not decrease to within the threshold of atmospheric pressure. In such an example, the fuel filler neck may be prevented from being accessed, or in other words, the refueling lock (e.g.  245 ) may be maintained locked. 
     Accordingly, proceeding to  325 , method  300  may include indicating whether the fuel filler neck was accessed manually or not. Specifically, in response to the refueling lock not opening due to pressure not decreasing to within the threshold of atmospheric pressure, a vehicle operator may manually unlock the refueling lock (which may for example comprise a fuel door lock), by depressing a manual refueling lock button (e.g.  191 ) positioned in a trunk of the vehicle, for example. The manual refueling lock is mentioned as being in the trunk, but may be anywhere in such a vehicle without departing from the scope of this disclosure. 
     Thus, in a case where the fuel filler neck is not indicated to be accessed manually, it may be understood that upon commanding open the FTIV, the fuel tank depressurized to within the threshold of atmospheric pressure thus resulting in the unlocking of the refueling lock to enable fuel to be added to the fuel system. Accordingly, in such an example, method  300  may proceed from  325  to  330 , where refueling may proceed. At  330 , the vehicle operator or gas station attendant may dispense a desired amount of fuel to the fuel tank, and afterward, with a desired amount of fuel having been added to the tank, method  300  may end. While not explicitly illustrated, it may be understood that in a case where the fuel system did not depressurize, but where the vehicle operator decides to drive off rather than manually open the refueling lock, then method  300  may be aborted. 
     Returning to  325 , if the fuel filler neck is indicated to have been accessed manually, method  300  may proceed to  335 . At  335 , method  300  may include monitoring fuel system pressure and fuel level during refueling of the fuel tank. Fuel system pressure and fuel level may be monitored such that a diagnosis of whether the FTPT has an inherent offset, whether the FTPT is stuck in range, or whether the FTIV is stuck closed or if other restrictions are present, may be indicated. Accordingly, such a diagnosis may be made via the lookup table at  FIG. 4 . It may be understood that the lookup table depicted at  FIG. 4  for diagnosing fuel system integrity depends on both the fuel system pressure and the fuel level during refueling of the fuel tank. 
     Accordingly, turning now to  FIG. 4 , an example lookup table  400  is depicted. Lookup table  400  includes an outcome  405  (outcomes A-C), symptoms  410  for each possible outcome, a diagnosis  415  based on the particular symptoms, and mitigating actions  420  that may be taken for each particular outcome. 
     Briefly, outcome A may include an indication that the FTPT sensor reports pressure decaying to atmospheric pressure just prior (e.g. less than 1 minute, or less than 30 seconds) to fuel level increasing, but where the ensuing refueling event is plagued with one or more premature shutoff events of a refueling dispenser that is dispensing fuel to the fuel tank. In such a case, the diagnosis may include a determination that the FTPT is functioning as desired, but that the FTIV is stuck closed (or that there is some other restriction preventing depressurization). More specifically, because the refueling lock was manually opened, with the refueling dispenser placed in the fuel filler neck, the fuel tank may at least partially become coupled to atmosphere. Thus, if the FTPT were functioning as desired, then once the refueling dispenser is placed in the fuel filler neck, pressure may be expected to rapidly approach atmospheric pressure. If the FTIV is stuck closed and/or if there is a restriction in the line coupling the fuel tank to the fuel vapor storage canister and/or a restriction in the vent line coupling the canister to atmosphere, then any attempt to refuel may be expected to result in premature shutoffs of the refueling dispenser. Accordingly, a rapid decay to atmospheric pressure upon insertion of the fuel dispenser into the fuel filler neck, followed by one or more premature shutoffs, may comprise outcome A, or in other words, an indication that the FTPT is functioning as desired or expected, and that the FTIV is stuck closed (or there is some other restriction in the lines coupling the fuel tank to atmosphere). 
     In such an example, mitigating action may include illuminating a malfunction indicator light (MIL) at the vehicle dash, to alert the vehicle operator of a request to service the vehicle. A diagnostic trouble code (DTC) may be set, and stored at the controller, indicating the issue of the stuck closed FTIV and/or restriction. Because of the inability to refuel without premature shutoffs, the vehicle may be operated in such a way as to reduce opportunity where the vehicle may have to be refueled. Specifically, the vehicle may be operated in electric-only mode as frequently as possible, until the issue has been resolved. Furthermore, any tests for undesired evaporative emissions on the fuel system and/or evaporative emissions system that rely on evacuating the fuel system and/or evaporative emissions system, may be discontinued until the issue is mitigated. 
     Outcome B depicts a situation where the refueling lock was manually opened, and where pressure in the fuel system does not change either just prior to or during the addition of fuel to the tank. The refueling event may proceed without premature shutoffs of the refueling dispenser, and fuel level may linearly increase over the course of the refueling event. In such an example, the FTPT may be stuck in range or in other words, not moving or registering any change in pressure, and thus may be understood to be degraded. Furthermore, for such an outcome, the FTIV may be indicated to be functioning as desired, and an absence of restrictions may be indicated in the lines that couple the fuel system to atmosphere. 
     Mitigating action for such an outcome B may include illuminating a MIL at the dash to alert the vehicle operator of a request to service the vehicle, and a DTC may be set related to the degraded FTPT, and may be stored at the controller. A flag may be set at the controller, indicating that refueling was requested, but that in order to do so, the refueling lock had to be manually unlocked. If such an event occurs more than a predetermined number of times (e.g. three or more) where the refueling lock has to be manually opened, then control strategy may be modified to enable the refueling lock to be unlocked in response to a request for refueling, regardless of pressure in the tank. In other words, in response to a request to refuel after the predetermined number of times the refueling lock was manually opened, the refueling lock may be opened without the vehicle operator having to manually open the lock. In other words, the controller may alter the strategy such that the request to refuel (e.g. pressing of button  197 ) may command unlocked the refueling lock, without reliance on fuel system depressurization. 
     Furthermore, because the FTPT is stuck in range, any tests for presence or absence of undesired evaporative emissions that rely on the FTPT, may be postponed until mitigating action is undertaken to resolve the issue with the FTPT. 
     Outcome C depicts a situation where the refueling event initiated with a particular offset that did not decay to atmospheric pressure just after inserting the refueling dispenser and prior to fuel level in the fuel tank rising. Instead, Outcome C depicts a situation where pressure rises to a steady state that is greater than the initial offset, and where fuel level in the fuel tank rises linearly without premature shutoffs. Accordingly, Outcome C is diagnosed as a situation where the FTPT has an inherent offset, but where there are no restrictions in the lines coupling the fuel tank to atmosphere and where the FTIV is not stuck closed. 
     With an inherent offset diagnosed as per outcome C, mitigating action may include illuminating a MIL at the dash, alerting the vehicle operator of a request to service the vehicle. A DTC may be set at the controller, reflecting the FTPT with the inherent offset. Mitigating action for outcome C may include setting a flag at the controller to reflect the inherent offset, such that if the refueling lock is manually unlocked a predetermined number of times (e.g. 3 or more), then the controller may alter its strategy to enable the refueling lock unlocked in response to a request to refuel, rather than having to manually unlock the refueling lock. In some examples, the refueling lock may be unlocked in response to a request to refuel provided that pressure in the fuel system is within a threshold (e.g. +/−5 InH2O) of the determined offset. For example, if the offset is determined to be 7 InH2O (where 7 InH2O as registered via the FTPT corresponds to atmospheric pressure due to the inherent offset), then the refueling lock may be unlocked responsive to pressure in the fuel system being anywhere from 2 InH2O to 12 InH2O. 
     Because for outcome C the FTPT offset is determined/known, the offset may be relied upon for any determination of a presence of absence of undesired evaporative emissions stemming from the fuel system. For example, if a fuel system pressure corresponding to 6 InH2O or greater (as an example) is used as an indication that a sealed fuel system is free from a source of undesired evaporative emissions, and there is an inherent offset of 8 InH2O, then when pressure is at 8 InH2O the fuel system may be indicated to be free from undesired evaporative emissions, when in fact there may be a source of undesired evaporative emissions stemming from the fuel system, if the offset is not accounted for. Accordingly, for outcome C, the inherent offset may be corrected for in the control strategy, such that (as per the above example) atmospheric pressure may correspond to the offset (8 InH2O), and thresholds for indicating a presence of undesired evaporative emissions may be correspondingly adjusted. For example, undesired evaporative emissions may be indicated if pressure in the sealed fuel system during a diagnostic for presence or absence of undesired evaporative emissions is 8 InH2O+/−6 InH2O. In other words, the inherent offset may become atmospheric pressure, and the thresholds may be set as +/−6 InH2O of atmospheric pressure (where atmospheric pressure corresponds to the inherent offset). 
     The above example relates to a situation where the fuel system is sealed (e.g. FTIV closed), and standing pressure greater than a threshold pressure (e.g. above 6 InH2O or below −6 InH2O) is used to indicate presence or absence of undesired evaporative emissions. However, there may be other tests for undesired evaporative emissions for which threshold(s) may be adjusted in response to a determination of an FTPT offset. In one example, a test for presence or absence of undesired evaporative emissions may include the fuel system and/or evaporative emissions system being evacuated via engine manifold vacuum, and in response to a predetermined threshold negative pressure being reached, the fuel system and/or evaporative emissions system may be sealed from atmosphere and a pressure bleed-up monitored. If pressure in the sealed fuel system and/or evaporative emissions system remains below a pressure bleed-up threshold (and/or if a pressure bleed-up rate is below a pressure bleed-up rate threshold), then an absence of undesired evaporative emissions may be indicated. An inherent offset of the FTPT may result in situations where a source of undesired evaporative emissions are indicated when in reality, the fuel system and/or evaporative emissions system is free from a source of undesired evaporative emissions. Alternatively, an inherent offset of the FTPT may result in situations where undesired evaporative emissions are not indicated, but should be, where the inherent offset is not accounted for. Thus, a determination of an inherent offset of the FTPT may be used to adjust the negative pressure threshold for evacuating the fuel system and/or evaporative emissions system, and may additionally be used to adjust the pressure bleed-up threshold (and pressure bleed-up rate threshold) for such types of test for undesired evaporative emissions. 
     Another example of a test for presence of absence of undesired evaporative emissions is based on heat rejection from an engine at a vehicle-off event, and is referred to (as discussed above) as an EONV test. As discussed, such a test includes a phase where a pressure build in the fuel system is monitored and if a positive pressure threshold is reached, then undesired evaporative emissions are not indicated. If the positive pressure threshold is not reached, then a vacuum-phase may be conducted where pressure in the fuel system is monitored as the fuel system cools and if a negative pressure threshold is reached, then undesired evaporative emissions are not indicated. Of course, an FTPT with an inherent offset may result in a vehicle fuel system being indicated to be free from undesired evaporative emissions when in fact, the positive pressure threshold and/or negative pressure threshold was only reached because of the offset. Thus, in such a case, if FTPT offset is used to appropriately adjust the positive pressure and negative pressure thresholds, then incorrect indications as related to presence or absence of undesired evaporative emissions may be avoided. 
     In regards to the EONV-type test described above, in a situation where FTPT offset is indicated to be 10 InH2O, then the positive pressure threshold may be increased by 10 InH2O, and the negative pressure threshold may be made less negative by 10InH2O. In this way, an accurate indication as to presence of absence of undesired evaporative emissions may be indicated for such a test diagnostic. 
     Returning to  FIG. 3 , upon diagnosing fuel system degradation based on fuel system pressure and fuel fill level at  340 , method  300  may proceed to  345 . At  345 , method  300  may include proceeding with refueling where possible. For example, outcome A may allow for a limited amount of refueling, as the refueling may be plagued with premature shutoffs. Alternatively, both outcomes B and C may allow for the fuel tank to be refueled without premature shutoffs. 
     Proceeding to step  350 , method  300  may include updating vehicle operating conditions and may further include taking mitigating action. For example, updating vehicle operating conditions may include setting MILs and/or DTCs, and may further include taking the mitigating action as detailed above at  FIG. 4 . Method  300  may then end. 
     Turning now to  FIG. 5A , an example timeline  500  is depicted, illustrating outcome A as depicted at  FIG. 4 , as determined via the method of  FIG. 3 . Timeline  500  includes plot  505 , indicating whether refueling is requested (yes or no), over time. It may be understood that a refuel request as per plot  505  includes a vehicle operator depressing a refueling button (e.g.  197 ) at the dash (e.g.  196 ). It may be further understood that such a request does not automatically result in the refueling lock (e.g.  245 ) being unlocked. Timeline  500  further includes plot  510 , indicating fuel system pressure as monitored via an FTPT (e.g.  291 ). Line  511  represents atmospheric pressure, and line  512  represents a predetermined threshold away from atmospheric pressure, where, if below, the fuel system may be indicated to be depressurized. While not explicitly illustrated, another predetermined threshold may be below atmospheric pressure, the same amount away from atmospheric pressure as line  512 . Timeline  500  further includes plot  515 , indicating fuel level in a fuel tank of the fuel system. Fuel level may be monitored via a fuel level indicator (FLI) (e.g.  234 ), and may increase (+) or decrease (−) over time. Timeline  500  further includes plot  520 , indicating a status of an FTIV (e.g.  252 ), over time. The FTIV may be either commanded fully open or fully closed, over time. Timeline  500  further includes plot  525 , indicating whether the refueling lock (e.g.  245 ) has been manually unlocked (yes or no), over time. As discussed above, manually unlocking the refueling lock may include depression of a manual refueling lock button (e.g.  191 ) positioned for example in a trunk of the vehicle. 
     At time t 0 , while not explicitly illustrated it may be understood that the vehicle is in motion, and is traveling to a refueling station. Accordingly, refueling has not yet been requested (plot  505 ), pressure in the fuel tank is above atmospheric pressure (plot  510 ), and fuel level in the tank is relatively low (plot  515 ). The FTIV is closed (plot  520 ), and the refueling lock has not been manually unlocked (plot  525 ). 
     At time t 1 , refueling is requested (plot  505 ). Accordingly, at time t 2 , the FTIV is commanded open to depressurize the fuel system, prior to enabling refueling to commence (prior to unlocking the refueling lock). However, between time t 2  and t 3 , pressure in the fuel system does not decline. With pressure in the fuel system not declining, it may be understood that the refueling lock remains locked. Accordingly, at time t 3 , the refueling lock is manually unlocked via the vehicle operator (plot  525 ). 
     In response to the refueling lock being manually unlocked, between time t 3  and t 4 , pressure in the fuel system decays to atmospheric pressure, represented by line  511 . Thus, between time t 3  and t 4  it may be understood that a refueling nozzle was inserted into the fuel filler neck, thus coupling the fuel system to atmosphere, and as a result pressure in the fuel system rapidly decayed to atmospheric pressure. Between time t 4  and t 5 , pressure in the fuel system steadily rises (plot  510 ) and fuel level increases (plot  515 ), yet at time t 5 , pressure in the fuel system peaks and then rapidly declines between time t 5  and t 6 . Furthermore, fuel level stabilizes and does not rise further between time t 5  and t 6 . Thus, it may be understood that at time t 5  a premature shutoff of the refueling dispenser occurred due to pressure in the fuel system rising to a point where it induced automatic shutoff of the refueling dispenser. 
     Between time t 6  and t 7 , pressure in the fuel system again rises and peaks (plot  510 ), and fuel is again dispensed into the fuel tank (plot  515 ). At time t 7 , another premature shutoff of the refueling dispenser is induced, thus between time t 7  and t 8  pressure in the fuel system again rapidly declines and fuel ceases to be added to the fuel tank. Between time t 8  and t 9 , pressure again rises in the fuel system (plot  510 ) and fuel is once again dispensed into the fuel tank (plot  515 ). At time t 9 , another premature shutoff of the refueling dispenser is induced, thus between time t 9  and t 10  pressure in the fuel system rapidly declines and fuel is no longer dispensed into the fuel tank. No further attempts at dispensing fuel into the fuel tank are attempted after time t 10 . With the fuel system at atmospheric pressure, the FTIV is commanded closed. While not explicitly illustrated, it may be understood that by time t 11 , the refueling lock is again locked, and thus refueling is no longer requested (plot  505 ) and the refueling lock is thus no longer indicated to be manually unlocked (plot  525 ). 
     Thus, timeline  500  indicates outcome A as depicted at  FIG. 4 , because it was observed that pressure in the fuel system rapidly decayed to atmospheric pressure between time t 3  and t 4  in response to the refueling dispenser being inserted into the fuel filler neck, and that subsequent attempts to refuel the fuel tank included a number of premature shutoffs of the refueling dispenser. Thus, such a timeline indicates a situation where the FTPT is functioning as desired but where the FTIV is either stuck closed or there is a restriction in one of the lines coupling the fuel system to atmosphere during the refueling event. 
     Turning now to  FIG. 5B , another example timeline  540  is depicted. Timeline  540  depicts example outcome B, as discussed above at  FIG. 4 . Timeline  540  includes plot  545 , indicating whether refueling is requested (yes or no), over time. It may be understood that a refuel request as per plot  545  includes a vehicle operator depressing a refueling button (e.g.  197 ) at the dash (e.g.  196 ). It may be further understood that such a request does not automatically result in the refueling lock (e.g.  245 ) being unlocked. Timeline  540  further includes plot  550 , indicating fuel system pressure as monitored via an FTPT (e.g.  291 ). Line  551  represents atmospheric pressure, and lines  552  and  553  represent predetermined thresholds away from atmospheric pressure, where, if pressure is below plot  553  and above plot  552 , the fuel system may be indicated to be depressurized. In other words, if pressure is between lines  552  and  553 , then the fuel system may be determined to be depressurized. Timeline  540  further includes plot  555 , indicating fuel level in a fuel tank of the fuel system. Fuel level may be monitored via a fuel level indicator (FLI) (e.g.  234 ), and may increase (+) or decrease (−) over time. Timeline  540  further includes plot  560 , indicating a status of an FTIV (e.g.  252 ), over time. The FTIV may be either commanded fully open or fully closed, over time. Timeline  540  further includes plot  565 , indicating whether the refueling lock (e.g.  245 ) has been manually unlocked (yes or no), over time. As discussed above, manually unlocking the refueling lock may include depression of a manual refueling lock button (e.g.  191 ) positioned for example in a trunk of the vehicle. 
     At time t 0 , while not explicitly illustrated it may be understood that the vehicle is in motion, and is traveling to a refueling station. Accordingly, refueling has not yet been requested (plot  545 ), pressure in the fuel tank is above atmospheric pressure (plot  550 ), and fuel level in the tank is relatively low (plot  555 ). The FTIV is closed (plot  560 ), and the refueling lock has not been manually unlocked (plot  565 ). 
     At time t 1 , refueling is requested (plot  545 ). Accordingly, at time t 2 , the FTIV is commanded open to depressurize the fuel system, prior to enabling refueling to commence (prior to unlocking the refueling lock). However, between time t 2  and t 3 , pressure in the fuel system does not decline. With pressure in the fuel system not declining, it may be understood that the refueling lock remains locked. Accordingly, at time t 3 , the refueling lock is manually unlocked via the vehicle operator (plot  525 ). 
     In response to the refueling lock being manually unlocked, between time t 3  and t 4  pressure in the fuel system does not decay to atmospheric pressure, represented by line  551 . However, at time t 4  fuel level in the fuel tank begins to rise, and between time t 4  and t 5 , fuel level in the fuel tank linearly increases. Thus, it may be understood that although the refueling dispenser was placed in the fuel filler neck thus coupling the fuel system to atmospheric pressure, pressure in the fuel system did not decay, and furthermore, pressure did not change upon fuel being added to the tank between time t 4  and t 5 . Because no premature shutoffs of the refueling dispenser are indicated between time t 4  and t 5 , the issue of why the fuel tank did not initially depressurize cannot be attributed to the FTIV being stuck closed, or to restrictions in the lines coupling the fuel system to atmosphere. Instead, the reason for the lack of depressurization of the fuel tank is because the FTPT is stuck in range as discussed above with regard to  FIG. 4 , outcome B. 
     Between time t 5  and t 6 , no further fuel is dispensed, and while not explicitly illustrated, it may be understood that the refueling lock is once again locked, and accordingly, refueling is no longer requested (plot  545 ), the FTIV is commanded closed (plot  560 ) and the refuel lock is no longer indicated to be manually unlocked (plot  565 ). 
     Turning now to  FIG. 5C , another example timeline  570  is depicted. Timeline  570  depicts example outcome C, as discussed above at  FIG. 4 . Timeline  570  includes plot  575 , indicating whether refueling is requested (yes or no), over time. It may be understood that a refuel request as per plot  575  includes a vehicle operator depressing a refueling button (e.g.  197 ) at the dash (e.g.  196 ). It may be further understood that such a request does not automatically result in the refueling lock (e.g.  245 ) being unlocked. Timeline  570  further includes plot  580 , indicating fuel system pressure as monitored via an FTPT (e.g.  291 ). Line  581  represents atmospheric pressure, and lines  582  and  583  represent predetermined thresholds away from atmospheric pressure, where, if below plot  583  or above plot  582 , the fuel system may be indicated to be depressurized. In other words, if pressure is between lines  582  and  583 , then the fuel system may be determined to be depressurized. Timeline  570  further includes plot  585 , indicating fuel level in a fuel tank of the fuel system. Fuel level may be monitored via a fuel level indicator (FLI) (e.g.  234 ), and may increase (+) or decrease (−) over time. Timeline  570  further includes plot  590 , indicating a status of an FTIV (e.g.  252 ), over time. The FTIV may be either commanded fully open or fully closed, over time. Timeline  570  further includes plot  595 , indicating whether the refueling lock (e.g.  245 ) has been manually unlocked (yes or no), over time. As discussed above, manually unlocking the refueling lock may include depression of a manual refueling lock button (e.g.  191 ) positioned for example in a trunk of the vehicle. 
     At time t 0 , while not explicitly illustrated it may be understood that the vehicle is in motion, and is traveling to a refueling station. Accordingly, refueling has not yet been requested (plot  575 ), pressure in the fuel tank is above atmospheric pressure (plot  580 ), and fuel level in the tank is relatively low (plot  585 ). The FTIV is closed (plot  590 ), and the refueling lock has not been manually unlocked (plot  595 ). 
     At time t 1 , refueling is requested (plot  575 ). Accordingly, at time t 2 , the FTIV is commanded open to depressurize the fuel system, prior to enabling refueling to commence (prior to unlocking the refueling lock). However, between time t 2  and t 3 , pressure in the fuel system does not decline. With pressure in the fuel system not declining, it may be understood that the refueling lock remains locked. Accordingly, at time t 3 , the refueling lock is manually unlocked via the vehicle operator (plot  595 ). 
     In response to the refueling lock being manually unlocked, between time t 3  and t 4 , pressure in the fuel system does not decay to atmospheric pressure, represented by line  551 . However, at time t 4  fuel level in the fuel tank begins to rise, and between time t 4  and t 5 , fuel level in the fuel tank linearly increases. Thus, it may be understood that although the refueling dispenser was placed in the fuel filler neck thus coupling the fuel system to atmospheric pressure, pressure in the fuel system did not decay. However, as fuel is dispensed into the fuel tank between time t 4  and t 5 , pressure in the fuel system increases from the value it was prior to fuel being added to the fuel tank, and stabilizes at a new higher pressure level (plot  580 ). At time t 5  it may be understood that the refueling dispenser is withdrawn from the fuel filler neck (without an automatic shutoff being induced). Thus, no further fuel is dispensed between time t 5  and t 6 , and pressure rapidly decays to the offset value the pressure was at prior to the fuel tank receiving fuel from the dispenser (plot  580 ). While not explicitly illustrated, the refueling lock is re-locked just prior to time t 6 . At time t 6 , refueling is no longer requested, the FTIV is commanded closed, and the refueling lock is no longer indicated to be manually unlocked. 
     Accordingly, timeline  540  illustrates a situation corresponding to outcome C, where there is an inherent offset in the FTPT, but otherwise the FTPT is functioning as desired, and there are no indications that the FTIV is stuck closed and/or that there are any restrictions in the lines coupling the fuel system to atmosphere. It may be understood that the offset corresponding to the amount above atmospheric pressure as read by the FTPT while the fuel system is coupled to atmospheric pressure (between time t 2 -t 4 ) may be used to compensate any tests for presence or absence of undesired evaporative emissions that rely on the FTPT. Furthermore, as discussed above the offset may be used to reconfigure control strategy for depressurizing the fuel system, such that the offset value may correspond to atmospheric pressure, and the fuel system may be indicated as being depressurized when pressure in the fuel system is within +/−5 InH2O of the offset value. This may enable the refueling lock to be unlocked in response to the request for refueling, without the vehicle operator having to manually unlock the refueling lock. In some examples, compensation whereby a request for refueling (without having to manually unlock the refueling lock) may be sufficient to command the refueling lock unlocked may be established after a predetermined number of times (e.g. 3) that the vehicle operator manually has to unlock the refueling lock to commence refueling. More specifically, after the predetermined number of times that the refueling lock has to be manually unlocked in response to a request to refuel, then control strategy may be modified such that the act of requesting refueling (e.g. pressing of button  197 ) may trigger the refueling lock unlocked, without reliance on the FTPT, in some examples. 
     While a refueling event may comprise an event whereby the FTPT may be rationalized and any inherent offset determined, and which may enable a determination as to whether a reason for a lack of fuel tank depressurization is likely due to a degraded FTPT or a degraded FTIV (or a restriction in one or more lines coupling the fuel tank to atmosphere), it is herein recognized that there may be other opportunities for determining FTPT offset. 
     Specifically, it is herein recognized that a refueling event may be a rare event for certain hybrid electric vehicles, such as plug-in hybrid electric vehicles, for example. Thus, it may not be desirable to rely solely on refueling events for rationalizing the FTPT and/or determining FTPT offset. However, because a sealed fuel tank is at most times under either positive or negative pressure with respect to atmospheric pressure, it may not be desirable to simply unseal the fuel system to rationalize the FTPT, as venting the fuel tank with positive pressure therein may undesirably load the canister with fuel vapors and as venting the fuel tank with negative pressure therein may undesirably draw in air into the fuel tank, which may disturb partial pressures and cause further vaporization. Drawing air into the tank may also prematurely age the fuel therein, resulting in the fuel losing its volatility which is undesirable particularly in a PHEV where fuel may remain in a tank for a prolonged period of time as the vehicle is regularly charged. 
     However, there are certain points during a diurnal cycle where pressure in a sealed fuel tank may be at atmospheric pressure, specifically during a transition from a heat gain portion of the diurnal cycle to a heat loss portion of the diurnal cycle. If the fuel system is unsealed at such a time, then neither undesirable loading of the canister nor the drawing in of air to the fuel system may occur, and the FTPT may be rationalized. 
     Turning now to  FIG. 6 , an example illustration of a diurnal cycle  600  as a graph of solar intensity and temperature as a function of the time of day, is shown. Incoming solar radiation  602  begins increasing at sunrise  604 , and rises to a maximum near mid-day before declining until sunset  606 . As such, sunrise  604  marks a time of day near where a heat gain cycle is at its greatest, and sunset  606  marks a time of day near where a heat loss cycle is at its greatest. Accordingly, ambient temperature  608  is shown, illustrating the increase in temperature from a minimum temperature  610  near sunrise  604 , and the decrease in temperature from a maximum temperature  612  prior to sunset  606 . As such, if a fuel system is sealed during the heat gain cycle, it may be expected that pressure may build in the sealed fuel system. Alternatively, if the fuel system is sealed during the heat loss cycle, then it may be expected that the outside temperatures may serve to reduce pressure in the sealed fuel system. 
     Accordingly, during the course of a diurnal cycle, there may be a point or points at which pressure in the sealed fuel system is at atmospheric pressure. Specifically, when a positive pressure exists in the fuel system, as the fuel system cools during the diurnal cycle, pressure may be reduced until the fuel system is at atmospheric pressure, just prior to transitioning to building a negative pressure in the sealed fuel system. Alternatively, when a negative pressure exists in the fuel system, as the fuel system warms during the diurnal cycle, pressure may become equivalent to atmospheric pressure just prior to transitioning to building a positive pressure in the sealed fuel system. However, determining exactly what point at which a sealed fuel system will be at atmospheric pressure may be challenging. To accomplish such a task, a wakeup circuit may be employed, which may wake a controller of the vehicle at precisely the time when pressure in the sealed fuel system is at atmospheric pressure. 
     Turning to  FIG. 7 , an example transfer function for a wakeup circuit (see  FIG. 8 ) is depicted. Graph  700  includes pressure along the horizontal axis (or x-axis) and volts along the vertical axis (or y-axis). Plot  705  depicts the example transfer function, which may be represented by the following equation:
 
Volts=0.1404*Pressure+2.5  (Eq. 1)
 
For example, if it is desired to wake the controller in order to conduct an FTPT offset determination at a point when pressure in a sealed fuel system is at atmospheric pressure (0), then a value of 2.5 volts may be calculated based on the transfer function (Eq. 1) above. This voltage may be applied via a latching chip on a comparator circuit (see  FIG. 8 ). The latching chip, in one example, may be an 8-bit latching chip. In the example, the 2.5 volts may be programmed into a digital/analog converter which applies the programmed voltage on the comparator circuit.
 
     Upon determining the zero pressure cross threshold (also referred to as atmospheric pressure) in volts, the controller may sleep while the comparator circuit is maintained awake. The comparator circuit may receive power from a battery of the vehicle. In one example, the comparator circuit may be a hot at all times (HAAT) comparator circuit. In this way, when outside temperatures due to the diurnal cycle are such that the sealed fuel system is at atmospheric pressure, as sensed for example via the FTPT (e.g.  291 ), the controller may be woken in order to conduct a diagnostic to determine FTPT offset. 
     Turning now to  FIG. 8 , an example comparator circuit  800  operable to wake the controller at a time following a vehicle-off condition where the sealed fuel system is at atmospheric pressure, is depicted. Comparator circuit  800  may include controller  212 . The controller may activate the comparator circuit  800  following a vehicle-off event when a request for FTPT offset determination is indicated. For a hybrid vehicle, a vehicle-off event may include the vehicle being turned off or deactivated completely. In some examples, vehicle-off may include a key-off event where the vehicle is powered off. 
     Controller  212  may include wake input  805 . Wake input  805  may be coupled to one or more inputs configured to wake up the controller when the controller is asleep following a vehicle-off condition. In response to a request to conduct a diagnostic to determine FTPT offset, the controller may determine the voltage corresponding to atmospheric pressure (see the transfer function depicted at  FIG. 7 ). While the controller is asleep, energy may be saved by shutting down on-board sensors, actuators, auxiliary components, diagnostics, etc. Essential functions, such as clocks and controller and battery maintenance operations may be maintained active during the sleep mode, but may be operated in a reduced power mode. During the awake mode, the controller may be operated at full power, and components regulated by the controller may be operated as dictated by engine and vehicle operating conditions. 
     Wake input  805  may be configured to trigger controller  212  to wake up when a signal is received indicating that pressure in the sealed fuel system is equal to the zero pressure cross threshold. Wake input  805  may be coupled to op-amp  815 . Op-amp  815  may comprise a first input  820 , a second input  825 , and an output  830 . In the depicted example, first input  820  is depicted as a positive input and second input  825  is configured as a negative input. In other examples, these configurations may be reversed. Thus, in the depicted configuration, the output  830  of comparator circuit  800  may be a difference between the first input  820  and the second input  825 . 
     In this configuration, wake input  805  is configured to wake controller  212  when a zero signal is received from output  830 . Op-amp  815  is configured to output a zero signal via output  830  when the value of a voltage at first input  820  is equal to the value of a voltage at second input  825 . First input  820  is coupled to FTPT  291 . The signal from the FTPT may be processed, for example band-pass filtered, via filter  835  before being communicated to op-amp  815 . In one example, filter  835  may be a low pass filter. Second input  825  is coupled to digital/analog output  840  from controller  212  via latching chip  845 . As such, once the controller determines the zero pressure cross threshold, the threshold is converted to a voltage reading (as described above with regard to  FIG. 7 ) and the voltage reading is latched via latching chip  845 . The controller may sleep after the latching chip latches the voltage reading. Thus, comparator circuit  800  may compare actual pressure of the sealed fuel system as sensed by the FTPT  291  to the threshold pressure (zero pressure cross threshold), such that the controller is awoken when pressure as monitored via the FTPT coincides with the pressure corresponding to the latched voltage reading. 
     In other words, as the output  830  of comparator circuit  800  is a difference between an output of the FTPT  291  (converted to voltage) and the latched voltage, the op-amp  815  may provide a zero signal as output  830  when pressure in the sealed fuel system reaches the zero pressure cross threshold. 
     Comparator circuit  800  may be configured as a HAAT circuit and as such, comparator circuit  800  may be a differential op-amp circuit. Comparator circuit  800  may receive power from voltage source  850 . Voltage source  850  may be a battery or other energy storage device and may be coupled to the comparator circuit via pull-up resistor  855 . 
     In this manner, the controller may be woken precisely at a time when it is expected that pressure in the sealed fuel system may be at atmospheric pressure. In this way, if the FTIV is commanded open, the canister with be neither loaded with fuel vapors stemming from the fuel system, nor will air be inducted into the fuel tank. 
     It may be understood that the sensor (e.g. FTPT) for which offset is being determined is also the sensor that is relied upon for waking the controller in order to conduct the diagnostic to determine offset. Thus, if an offset is indicated, then it may be understood that the new zero pressure cross threshold may correspond to the offset pressure. For example, if the offset of the FTPT is determined to be 1 InH2O, then using the transfer function (Eq. 1) discussed above, the voltage at which the comparator circuit may latch may comprise 2.6404 Volts. In this way, the controller may be woken up in order to learn FTPT offset, even under conditions where there already is an offset determined. Comparator circuit may be used in conjunction with method  900 , discussed in detail below, in order to conduct environmentally friendly FTPT offset diagnostics. 
     Thus, as discussed herein, a system for a hybrid vehicle may comprise a fuel system that includes a fuel tank and a fuel tank pressure transducer. The system may further comprise a fuel tank isolation valve in a conduit coupling the fuel system to a fuel vapor storage canister, the fuel vapor storage canister further coupled to atmosphere. The system may further comprise a comparator circuit that includes a first input voltage to an operational amplifier related to pressure in the fuel system as monitored via the fuel tank pressure transducer and a second input voltage that comprises a latched voltage corresponding to atmospheric pressure. The system may further comprise a controller with computer readable instruction stored on non-transitory memory that, when executed during a vehicle-off condition, cause the controller to in response to the first input voltage equaling the second input voltage, wake the controller and command the fuel tank isolation valve to a fully open position. The controller may store further instructions to monitor a pressure change in the fuel system in response to the commanding open the fuel tank isolation valve. The controller may store further instructions to set an offset of the fuel tank pressure transducer as a function of the pressure change in the fuel system in response to the commanding open the fuel tank isolation valve. The controller may store further instructions to adjust one or more thresholds for diagnostic routines that rely on output from the fuel tank pressure transducer. 
     In such a system, the controller may store further instructions to set the second input voltage based on a transfer function stored at the controller that transforms pressure to voltage. 
     In such a system, the controller may store further instructions to, in response to determining the offset, set the second input voltage as a function of a difference between the offset and atmospheric pressure. 
     Turning now to  FIG. 9 , an example method  900  for conducting an FTPT offset diagnostic is depicted. Specifically, method  900  may be used to determine FTPT offset, in response to a comparator circuit (e.g.  800 ) waking the controller (e.g.  212 ) to conduct the diagnostic when it is expected that pressure in the sealed fuel system will be at or near atmospheric pressure. 
     Method  900  will be described with reference to the systems described herein and shown in  FIGS. 1-2  and  FIG. 8 , though it should be understood that similar methods may be applied to other systems without departing from the scope of this disclosure. Method  900  may be carried out by a controller, such as controller  212  in  FIG. 2 , and may be stored at the controller as executable instructions in non-transitory memory. Instructions for carrying out method  900  and the rest of the methods included herein may be executed by the controller based on instructions stored on a memory of the controller and in conjunction with signals received from sensors of the engine system, such as the sensors described above with reference to  FIGS. 1-2  and  FIG. 8 . The controller may employ actuators such as FTIV (e.g.  252 ), etc., to alter states of devices in the physical world according to the method depicted below. 
     Method  900  begins at  905 , and includes estimating and/or measuring vehicle operating conditions. Operating conditions may be estimated, measured, and/or inferred, and may include one or more vehicle conditions, such as vehicle speed, vehicle location, etc., various engine conditions, such as engine status, engine load, engine speed, A/F ratio, manifold air pressure, etc., various fuel system conditions, such as fuel level, fuel type, fuel temperature, etc., various evaporative emissions system conditions, such as fuel vapor canister load, fuel tank pressure, etc., as well as various ambient conditions, such as ambient temperature, humidity, barometric pressure, etc. 
     Proceeding to  910 , method  900  may include indicating whether a key-off event is indicated. In this example method, a key-off event may be understood to comprise a vehicle-off event, where the vehicle is deactivated. If a key-off event is not indicated, then method  900  may proceed to  912 , where the vehicle may be operated according to driver demand. For example, the vehicle may be propelled via the engine, motor, or some combination of the two. Method  900  may then end. 
     Returning to  910 , in response to a key-off event being indicated, method  900  may proceed to  915 . At  915 , method  900  may include latching the zero pressure threshold via the comparator circuit (e.g.  800 ). For example, if no offset of the FTPT has been previously indicated, then the zero pressure cross threshold may comprise 2.5 V, as per the transfer function discussed above at  FIG. 7 . Alternatively, if there is already an indication that the FTPT offset is 2 InH2O, for example, then the zero pressure cross threshold may comprise 2.7808 V, as per the transfer function discussed above at  FIG. 7 . 
     With the zero pressure cross threshold latched via the comparator circuit, at  915  the controller may be put to sleep where essential functions are maintained powered at a reduced level, and where non-essential functions are powered off. Power to the comparator circuit may be maintained. 
     Proceeding to  920 , method  900  may include indicating whether a controller wakeup event is indicated. In other words, at  920 , method  900  determines whether the comparator circuit has triggered the waking of the controller (e.g.  212 ). If not, method  900  may proceed to  925 , where it is indicated as to whether a key-on event is indicated. For example, a vehicle operator may desire to travel via the vehicle, and thus may enter and activate the vehicle prior to the controller being woken to conduct the diagnostic for learning FTPT offset. In some examples, a key-on event may comprise a remote start event, for example. Accordingly, if a key-on event is indicated at  925 , method  900  may proceed to  930 , where the vehicle may be operated according to driver demand, as discussed above at  912 . Method  900  may then end. 
     Returning to  925 , if a key-on event is not indicated, method  900  may return to  920  where it is assessed as to whether the controller wake event has occurred. In response to the controller being triggered to wake at  920 , method  900  may proceed to  935 . At  935 , method  900  may include commanding open the FTIV, and may further include allowing for system stabilization. Specifically, while the comparator circuit is designed for waking the controller and commanding the fuel system unsealed at a point where pressure in the fuel system is at atmospheric pressure, depending on any inherent FTPT offset, pressure in the fuel system may not be precisely at atmospheric pressure. In other words, there may be some amount of pressure or vacuum in the fuel system that gets relieved upon commanding open the FTIV. 
     With the fuel system commanded unsealed at  935 , method  900  may proceed to  940 . At  940 , method  900  may include recording fuel system pressure via the FTPT (e.g.  291 ). In this way, at step  945 , method  900  may include determining FTPT offset. For example, consider a situation where the comparator circuit was latched to wake the controller when pressure in the fuel system reads 0 InH2O. Accordingly, when the FTPT reads 0 InH2O, the controller may be woken, and the FTIV may be commanded open. However, when coupled to atmosphere, the FTPT may read 1 InH2O, for example. In this example, the offset of the FTPT would be determined at  945  to be 1 InH2O. Other examples are possible, such as when the fuel system is coupled to atmosphere, pressure as monitored via the FTPT may read −2 InH2O. In such an example, the offset of the FTPT would be determined at  945  to be −2 InH2O. However, while such methodology may not enable unsealing of the fuel system at precisely a time when pressure in the sealed fuel system is equal to atmospheric pressure, the methodology may reduce an overall amount of fuel vapors that may load the canister and/or may reduce induction of air into the fuel tank, as compared to a situation where such a wakeup circuit were not utilized. For example, unsealing the fuel system when pressure in the sealed fuel system is 1 InH2O will load the canister much less than a case where the fuel system is unsealed when pressure in the sealed fuel system is 30 InH2O or greater. Thus, by using the wakeup circuit as discussed, the unsealing of the fuel system may be conducted at times when it is likely or expected that pressure in the sealed fuel system will be near atmospheric pressure. 
     With the offset determined at  945 , method  900  may proceed to  950 . At  950 , method  900  may include commanding closed the FTIV to once again seal the fuel system from atmosphere. Proceeding to  955 , method  900  may include updating vehicle operating conditions. At  955 , updating vehicle operating conditions may include updating the FTPT offset at the controller. For example, if the FTPT offset is 1 InH2O, then such an offset may be stored at the controller such that controller strategy may rely on the offset value. In one example, as discussed, the offset may be set as the zero pressure cross threshold for any subsequent times where the comparator circuit (see  FIG. 8 ) is utilized to wake the controller in order to determine the FTPT offset. In another example, thresholds for a presence or absence of undesired evaporative emissions tests may be adjusted as a function of the offset value. For example, if the offset value is 10 InH2O, and the fuel system is considered to be free from a source of undesired evaporative emissions when the pressure is at or above 9 InH2O, then if not compensated for, the fuel system may be determined to be free from undesired evaporative emissions when in fact, the fuel system includes a source of undesired evaporative emissions. Thus, thresholds for indicating presence or absence of undesired evaporative emissions may be adjusted as a function of the determined offset of the FTPT. For detailed explanation of how thresholds may be adjusted for particular diagnostics for presence or absence of undesired evaporative emissions, refer to the description of  FIG. 3 , which similarly applies to the method of  FIG. 9 . 
     In some examples, the offset may be used to compensate fuel system depressurization determination. For example, in response to a request for refueling, the refueling lock (e.g.  245 ) may only unlock in response to pressure in the fuel system being within an absolute value threshold of atmospheric pressure. Thus, an FTPT that has an inherent offset that is outside the range of the absolute value threshold, may not enable unlocking of the refueling lock unless the refueling lock is manually unlocked. Thus, in some examples, the determined FTPT offset may be used to enable access to a fuel filler neck (or in other words, unlocking of the refueling lock), even under conditions where the FTPT does not indicate pressure in the fuel system within the range bounded by the absolute value thresholds of atmospheric pressure. 
     For example, in some cases, in response to an indication that the FTPT offset is such that the refueling lock may not open in response to fuel system depressurization, then the controller strategy may be updated to unlock the refueling lock in response to pressure in the fuel system being within a threshold (e.g. +/−5 InH2O) of the FTPT offset value. For example, if the FTPT offset is 10 InH2O, then control strategy may unlock the refueling lock in response to pressure in the fuel system being between 5 and 15 InH2O. In another example where the FTPT offset is such that control strategy may not unlock the refueling lock in response to fuel system depressurization, the controller strategy may be updated to enable the refueling lock unlocked regardless of the pressure reading in the fuel system once coupled to atmosphere for refueling. In some examples, control strategy may only be updated as discussed, in response to a predetermined threshold number of times the FTPT offset has been determined. For example, if an FTPT offset has been indicated three times or more, in other words three independent diagnostics that rely on the comparator circuit have indicated the FTPT offset, then control strategy may be modified to enable the refueling lock to be opened in response to fuel system depressurization as discussed above. 
     Proceeding to  960 , method  900  may include once again sleeping the controller. Method  900  may then end. 
     Thus, as discussed herein, a method may comprise waking a controller of a vehicle when it is expected that a pressure in a fuel system that is sealed from atmosphere is at atmospheric pressure, unsealing the fuel system, and indicating an offset of a pressure sensor used to monitor the pressure in the fuel system based on a pressure change as indicated via the pressure sensor after the fuel system is unsealed. 
     In such a method, unsealing the fuel system when the pressure in the fuel system is expected to be at atmospheric pressure reduces an amount by which a fuel vapor storage canister is loaded with fuel vapors from the fuel system and/or reduces an amount of air inducted into the fuel system in response to the unsealing, as compared to unsealing the fuel system at a time when the pressure in the fuel system is not at atmospheric pressure. 
     In such a method, the offset of the pressure sensor may comprise an amount of the pressure change as indicated via the pressure sensor after the fuel system is unsealed. 
     In such a method, the offset may be used via the controller to adjust one or more thresholds for indicating a presence or an absence of undesired evaporative emissions stemming from at least the fuel system. 
     In such a method, the offset may be used via the controller to adjust a fuel system depressurization threshold at which the fuel system is indicated to be depressurized. 
     In such a method, the offset may be used via the controller to enable a refueling lock to be opened in response to a request for refueling the fuel system, without the refueling lock being manually opened. 
     In such a method, waking the controller may be via a comparator circuit that relies on input from the pressure sensor used to monitor the pressure in the fuel system. 
     In such a method, waking the controller when it is expected that the pressure in the fuel system is at atmospheric pressure is a function of the indicated offset of the pressure sensor. 
     In such a method, in response to the controller not waking during a period of time where the controller is expected to have been woken, scheduling a follow-up test to determined whether the pressure sensor is stuck and is not responding to differences in fuel system pressure. 
     In such a method, in response to the indication of an absence of the offset observed after a predetermined number of times that the controller is woken to indicate the offset, scheduling a follow-up test to determine whether the fuel system is not unsealing as expected in response to the waking the controller when it is expected that the pressure in the fuel system is at atmospheric pressure. In other words, the indication of an absence of an offset may be indicative of a situation where the fuel system is not unsealing, and as such, it may be desirable to assess such a possibility. It may be understood that scheduling the follow-up test may include ascertaining whether the fuel system depressurizes in response to a refueling event, as discussed above, or if the refueling lock has to be manually unlocked in order to commence the refueling event. In this way it may be determined as to whether the indication of the absence of the offset is real. 
     Another example of a method comprises waking a controller of a vehicle when a pressure in a fuel system that is sealed from atmosphere is transitioning from a positive pressure to a negative pressure with respect to atmospheric pressure, or vice versa, the waking a function of the pressure as monitored via a fuel tank pressure transducer positioned in the fuel system; and unsealing the fuel system to couple the fuel system to atmosphere in order to determine an offset of the fuel tank pressure transducer. 
     In such a method, waking as the function of the pressure as monitored via the fuel tank pressure transducer may be via a comparator circuit that wakes the controller when a first input to the comparator circuit that comprises a first voltage corresponding to a first pressure readout from the fuel tank pressure transducer is equivalent to a second input to the comparator circuit that comprises a second latched voltage for the comparator circuit such that a zero signal is output from the comparator circuit. In response to the offset being determined and where the offset is a non-zero quantity, the method may include setting the second input to the comparator circuit as a function of atmospheric pressure plus the offset for subsequently waking the controller of the vehicle. The second latched voltage may be determine via a transfer function that determines the second latched voltage as a function of the pressure when the pressure in the fuel system is transitioning from the positive pressure to the negative pressure or vice versa. 
     In such a method, determining the offset may include monitoring a pressure change direction and magnitude as indicated via the fuel tank pressure transducer in response to the unsealing of the fuel system, and setting the offset as a function of the pressure change direction and magnitude. 
     In such a method, the method may further comprise in response to the offset being outside of a first threshold pressure range for indicating that the fuel system is depressurized, adjusting the first threshold pressure range to a second threshold pressure range that is a function of the offset to indicate that the fuel system is depressurized when the pressure in the fuel system is within the second threshold pressure range. 
     In such a method, the method may further comprise adjusting a pressure threshold for indicating a presence of a source of undesired evaporative emissions stemming from the fuel system, as a function of the offset. 
     Turning now to  FIG. 10 , an example timeline  1000  is shown, depicting how a wakeup circuit such as the wakeup circuit depicted at  FIG. 8  may be used to wake a controller of a vehicle at a time when pressure in a sealed fuel system corresponds to a particular pressure that is latched via the wakeup circuit, such that a diagnostic for FTPT offset may be conducted. Timeline  1000  includes plot  1005 , indicating controller status, over time. The controller may be either asleep, or awake, over time. Timeline  1000  further includes plot  1010 , indicating pressure in the fuel system as monitored via an FTPT (e.g.  291 ), over time. There are various thresholds associated with plot  1010  which will be discussed in further detail below. Timeline  1000  further includes plot  1025 , indicating whether the FTIV (e.g.  252 ) is open or closed, over time. 
     At time t 0 , the controller is asleep (plot  1005 ). The comparator circuit (e.g.  800 ) is powered, and the zero pressure cross threshold has been determined and corresponds to atmospheric pressure (0 InH2O). In other words, with the zero pressure cross threshold determined to be atmospheric pressure, this threshold has been converted to a voltage reading, and the voltage reading is thus latched via the latching chip (e.g.  845 ) of the comparator circuit. Thus, in this example timeline  1000 , when the FTPT registers atmospheric pressure, the controller may be woken up in order to determine FTPT offset. Thus, the zero pressure cross threshold is represented by dashed line  1011 . Furthermore, at time t 0 , the FTIV is closed, thus the fuel system is sealed from atmosphere. 
     At time t 1 , the controller is triggered to awake mode (plot  1005 ), as pressure as monitored via the FTPT reaches the zero pressure cross threshold represented by line dashed line  1011 . Accordingly, with the controller triggered awake, at time t 2  the FTIV is commanded open (plot  1025 ), to couple the fuel system to atmosphere. Just after the FTIV is commanded open, pressure in the fuel system rises slightly and stabilizes between time t 2  and t 3 . The pressure that the FTPT rises to represents the FTPT offset. Specifically, arrow  1013  points to dashed line  1012 , which comprises a new zero pressure cross threshold for the comparator circuit. In other words, because when the fuel system was coupled to atmosphere, pressure as monitored via the FTPT rose slightly above atmospheric pressure, this new value representing the offset becomes the new or second zero pressure cross threshold for the comparator circuit, such that subsequent FTPT offset diagnostics may be conducted using the second zero pressure cross threshold. The difference between atmospheric pressure and the new FTPT offset is represented by  1020 . 
     At time t 3 , with the FTPT offset having been determined, the FTIV is commanded closed, and then the controller is put to sleep at time t 4 . Between time t 4  and t 5 , pressure in the fuel system becomes negative with respect to atmospheric pressure. Thus, it may be understood that the vehicle is in a heat loss portion of the diurnal cycle. 
     Between time t 5  and t 6 , some time passes. It may be understood that between time t 5  and t 6 , the vehicle may be driven, but another FTPT offset diagnostic is not conducted. By time t 6 , the vehicle is returned to sleep mode, with the pressure as represented by dashed line  1012  used in conjunction with the transfer function discussed at  FIG. 7 , to latch a voltage reading corresponding to the pressure represented by dashed line  1012  via the latching chip of the comparator circuit, such that the controller may be woken up in response to pressure in the sealed fuel system reaching the pressure represented by dashed line  1012 . 
     Between time t 6  and t 7 , pressure in the sealed fuel system rises, thus it may be understood that the vehicle is in a heat gain portion of the diurnal cycle. At time t 7 , pressure in the sealed fuel system as monitored via the FTPT reaches the second zero pressure cross threshold, represented by dashed line  1012 . Accordingly, the controller is woken up (plot  1005 ), and at time t 8 , the FTIV is commanded open. After commanding open the FTIV, the FTPT reading changes slightly, becoming slightly more positive than the second zero pressure cross threshold. Thus, the pressure that the FTPT registers between time t 8  and t 9  represents a further FTPT offset. Specifically, arrow  1017  points to dashed line  1016 , which comprises another new or third zero pressure cross threshold for the comparator circuit. In other words, because when the fuel system was coupled to atmosphere at time t 8 , pressure as monitored via the FTPT rose slightly higher than the second zero pressure cross threshold, this new value becomes the third zero pressure threshold for the comparator circuit, such that subsequent FTPT offset diagnostics may be conducted using the third zero pressure cross threshold. The difference between the second zero pressure cross threshold  1012  and the third zero pressure cross threshold is represented by  1014 . The difference between the third zero pressure cross threshold and atmospheric pressure is represented by  1015 . Thus, in this example timeline, by time t 9 , FTPT offset has become the difference between the third zero pressure cross threshold  1016  and atmospheric pressure (also referred to in this example timeline as the zero pressure cross threshold  1011 ). 
     At time t 9 , the FTIV is commanded closed (plot  1025 ) thus sealing the fuel system from atmosphere. At time t 10 , the controller is once again slept, after having latched the third zero pressure cross threshold at the comparator circuit. Between time t 10  and t 11 , pressure in the sealed fuel system continues to rise (plot  1010 ). Thus, it may be understood that the vehicle is in a heat gain portion of the diurnal cycle. 
     Returning to  FIG. 9 , it is herein recognized that in a situation where a vehicle-off condition lasts for a duration where it is expected that the pressure in the fuel system may transition from a positive pressure to a negative pressure, or vice versa, but where the controller was not woken up, it may be that the FTPT is stuck in range, and is not registering fuel system pressure changes. Whether or not a wakeup is expected may be based on knowledge of the diurnal cycle, for example the controller of the vehicle may communicate with one or more servers, the internet, etc., and/or may retrieve information from the onboard GPS, related to current and future weather conditions, time of day, etc. In this way, it may be determined if a wakeup of the controller is expected for a vehicle-off condition. 
     If the controller is not awoken a predetermined number of times where the controller is expected to have awoken if the FTPT were functioning as desired, then control strategy may request a diagnostic such as that discussed above at  FIG. 3 . For example, in response to a request to refuel, if the fuel system does not depressurize to enable refueling, it may be likely that the FTPT is stuck in range. By conducting method  300  depicted at  FIG. 3 , the source of why the fuel system is not depressurizing (and by association, why the controller is not being woken up when otherwise expected to), may be determined. 
     Returning again to  FIG. 9 , it may be understood that in some examples, the controller may regularly be woken when expected to (e.g. when vehicle-off duration is sufficient for pressure in the sealed fuel system to transition from positive pressure to negative pressure, or vice versa), but no offset may be indicated. While such an indication may be due to the FTPT actually having no offset, such a result may alternatively be due to the FTIV not actually opening as desired and/or due to a presence of a restriction that does not enable the fuel system depressurized. Thus, similar to that discussed above, if a predetermined number (e.g. 3) of FTPT rationalization events that rely on waking the controller have occurred, without an offset being indicated, then control strategy may request the method of  FIG. 3  be conducted. In other words, if no FTPT offset is being indicated because the FTIV is not opening or due to a restriction in one or more lines coupling the fuel system to atmosphere, then in response to a request for refueling, if the fuel system does not depressurize, a likely reason is that the FTIV is stuck closed and/or there are restrictions in the one or more lines. In this example, by conducting the method of  FIG. 3 , it may be conclusively determined as to whether there is no inherent offset of the FTPT or if the FTIV is stuck closed and/or that there are restriction(s) in the one or more lines coupling the fuel system to atmosphere. 
     In this way, inherent offset of an FTPT may be determined, such that routines that involve fuel system depressurization and/or tests for undesired evaporative emissions may be improved. For example, thresholds for tests for undesired evaporative emissions may be adjusted in response to learning FTPT offset, and in cases where FTPT offset is such that a refueling lock may not unlock in response to fuel system depressurization due to the offset, control strategy may be updated to enable the refueling lock unlocked without having to manually unlock the refueling lock. Furthermore, the systems and methods disclosed herein may enable a determination as to whether the FTPT is stuck in range and not responding to pressure changes in the fuel system, and/or whether the FTIV which seals the fuel system is stuck closed or if there is some restriction in one or more lines coupling the fuel system to atmosphere which may prevent depressurization. 
     A technical effect is to recognize that when a refueling lock is manually opened due to the refueling lock not being opened in response to fuel system depressurization, when a refueling dispenser is placed in a fuel filler neck, the fuel system is coupled to atmosphere via the fuel filler neck. Accordingly, such a situation enables a determination as to whether the FTPT degradation (e.g. offset, stuck in range) is the reason for the refueling lock not opening, or if the reason stems from blockage in one or more lines coupling the fuel system to atmosphere (not including the fuel filler neck). Thus, a technical effect is to recognize that, responsive to the refueling lock being manually opened, such a determination as discussed above may be made based on fuel system pressure and fuel system fuel level during an ensuing refueling event. 
     Another technical effect is to recognize that FTPT rationalization may be conducted at times when the sealed fuel system is expected to be at atmospheric pressure, which may thus enable rationalization without undesirably loading the fuel vapor storage canister with vapors, and/or without inducting air into the fuel system, which may disturb partial pressures, increase vaporization, and decrease volatility. Thus, a technical effect is to recognize that a wakeup circuit that relies on the FTPT may be utilized in order to wake a controller at precisely a time when fuel system pressure is expected to be at atmospheric pressure, in order to conduct the FTPT rationalization procedure. 
     The systems and methods discussed herein may enable one or more systems and one or more methods. In one example, a method comprises waking a controller of a vehicle when it is expected that a pressure in a fuel system that is sealed from atmosphere is at atmospheric pressure; unsealing the fuel system; and indicating an offset of a pressure sensor used to monitor the pressure in the fuel system based on a pressure change as indicated via the pressure sensor after the fuel system is unsealed. In a first example of the method, the method further includes wherein unsealing the fuel system when the pressure in the fuel system is expected to be at atmospheric pressure reduces an amount by which a fuel vapor storage canister is loaded with fuel vapors from the fuel system and/or reduces an amount of air inducted into the fuel system in response to the unsealing, as compared to unsealing the fuel system at a time when the pressure in the fuel system is not at atmospheric pressure. A second example of the method optionally includes the first example, and further includes wherein the offset of the pressure sensor is an amount of the pressure change as indicated via the pressure sensor after the fuel system is unsealed. A third example of the method optionally includes any one or more or each of the first and second examples, and further includes wherein the offset is used via the controller to adjust one or more thresholds for indicating a presence or an absence of undesired evaporative emissions stemming from at least the fuel system. A fourth example of the method optionally includes any one or more or each of the first through third examples, and further includes wherein the offset is used via the controller to adjust a fuel system depressurization threshold at which the fuel system is indicated to be depressurized. A fifth example of the method optionally includes any one or more or each of the first through fourth examples, and further includes wherein the offset is used via the controller to enable a refueling lock to be opened in response to a request for refueling the fuel system, without the refueling lock being manually opened. A sixth example of the method optionally includes any one or more or each of the first through fifth examples, and further includes wherein waking the controller is via a comparator circuit that relies on input from the pressure sensor used to monitor the pressure in the fuel system. A seventh example of the method optionally includes any one or more or each of the first through sixth examples, and further includes wherein waking the controller when it is expected that the pressure in the fuel system is at atmospheric pressure is a function of the indicated offset of the pressure sensor. An eighth example of the method optionally includes any one or more or each of the first through seventh examples, and further includes wherein in response to the controller not waking during a period of time where the controller is expected to have been woken, scheduling a follow-up test to determine whether the pressure sensor is stuck and is not responding to differences in fuel system pressure. A ninth example of the method optionally includes any one or more or each of the first through eighth examples, and further includes wherein in response to the indication of an absence of the offset observed after a predetermined number of times that the controller is woken to indicate the offset, scheduling a follow-up test to determine whether the fuel system is not unsealing as expected in response to the waking the controller when it is expected that the pressure in the fuel system is at atmospheric pressure. 
     Another example of a method comprises waking a controller of a vehicle when a pressure in a fuel system that is sealed from atmosphere is transitioning from a positive pressure to a negative pressure with respect to atmospheric pressure, or vice versa, the waking a function of the pressure as monitored via a fuel tank pressure transducer positioned in the fuel system; and unsealing the fuel system to couple the fuel system to atmosphere in order to determine an offset of the fuel tank pressure transducer. In a first example of the method, the method further includes wherein the waking as the function of the pressure as monitored via the fuel tank pressure transducer is via a comparator circuit that wakes the controller when a first input to the comparator circuit that comprises a first voltage corresponding to a first pressure readout from the fuel tank pressure transducer is equivalent to a second input to the comparator circuit that comprises a second latched voltage for the comparator circuit such that a zero signal is output from the comparator circuit. A second example of the method optionally includes the first example, and further includes wherein in response to the offset being determined and where the offset is a non-zero quantity, setting the second input to the comparator circuit as a function of atmospheric pressure plus the offset for subsequently waking the controller of the vehicle. A third example of the method optionally includes any one or more or each of the first and second examples, and further includes wherein the second latched voltage is determined via a transfer function that determines the second latched voltage as a function of the pressure when the pressure in the fuel system is transitioning from the positive pressure to the negative pressure or vice versa. A fourth example of the method optionally includes any one or more or each of the first through third examples, and further includes wherein determining the offset includes monitoring a pressure change direction and magnitude as indicated via the fuel tank pressure transducer in response to the unsealing of the fuel system, and setting the offset as a function of the pressure change direction and magnitude. A fifth example of the method optionally includes any one or more or each of the first through fourth examples, and further comprises in response to the offset being outside of a first threshold pressure range for indicating that the fuel system is depressurized, adjusting the first threshold pressure range to a second threshold pressure range that is a function of the offset to indicate that the fuel system is depressurized when the pressure in the fuel system is within the second threshold pressure range. A sixth example of the method optionally includes any one or more or each of the first through fifth examples, and further comprises adjusting a pressure threshold for indicating a presence of a source of undesired evaporative emissions stemming from the fuel system, as a function of the offset. 
     An example of a system for a hybrid vehicle comprises a fuel system that includes a fuel tank and a fuel tank pressure transducer; a fuel tank isolation valve in a conduit coupling the fuel system to a fuel vapor storage canister, the fuel vapor storage canister further coupled to atmosphere; a comparator circuit that includes a first input voltage to an operational amplifier related to pressure in the fuel system as monitored via the fuel tank pressure transducer and a second input voltage that comprises a latched voltage corresponding to atmospheric pressure; and a controller with computer readable instructions stored on non-transitory memory that when executed during a vehicle-off condition, cause the controller to: in response to the first input voltage equaling the second input voltage, wake the controller and command the fuel tank isolation valve to a fully open position; monitor a pressure change in the fuel system in response to the commanding open the fuel tank isolation valve; set an offset of the fuel tank pressure transducer as a function of the pressure change in the fuel system in response to the commanding open the fuel tank isolation valve; and adjust one or more thresholds for diagnostic routines that rely on output from the fuel tank pressure transducer. In a first example of the system, the system further includes wherein the controller stores further instructions to set the second input voltage based on a transfer function stored at the controller that transforms pressure to voltage. A second example of the system optionally includes the first example, and further includes wherein the controller stores further instructions to, in response to determining the offset, set the second input voltage as a function of a difference between the offset and atmospheric pressure. 
     Note that the example control and estimation routines included herein can be used with various engine and/or vehicle system configurations. The control methods and routines disclosed herein may be stored as executable instructions in non-transitory memory and may be carried out by the control system including the controller in combination with the various sensors, actuators, and other engine hardware. The specific routines described herein may represent one or more of any number of processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various actions, operations, and/or functions illustrated may be performed in the sequence illustrated, in parallel, or in some cases omitted. Likewise, the order of processing is not necessarily required to achieve the features and advantages of the example embodiments described herein, but is provided for ease of illustration and description. One or more of the illustrated actions, operations and/or functions may be repeatedly performed depending on the particular strategy being used. Further, the described actions, operations and/or functions may graphically represent code to be programmed into non-transitory memory of the computer readable storage medium in the engine control system, where the described actions are carried out by executing the instructions in a system including the various engine hardware components in combination with the electronic controller. 
     It will be appreciated that the configurations and routines disclosed herein are exemplary in nature, and that these specific embodiments are not to be considered in a limiting sense, because numerous variations are possible. For example, the above technology can be applied to V-6, I-4, I-6, V-12, opposed 4, and other engine types. The subject matter of the present disclosure includes all novel and non-obvious combinations and sub-combinations of the various systems and configurations, and other features, functions, and/or properties disclosed herein. 
     As used herein, the term “approximately” is construed to mean plus or minus five percent of the range unless otherwise specified. 
     The following claims particularly point out certain combinations and sub-combinations regarded as novel and non-obvious. These claims may refer to “an” element or “a first” element or the equivalent thereof. Such claims should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and sub-combinations of the disclosed features, functions, elements, and/or properties may be claimed through amendment of the present claims or through presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, also are regarded as included within the subject matter of the present disclosure.