Abstract:
A method for fault identification of gas sensors exposed to a gas mixture is disclosed for gas sensors having an output that depends on concentrations of two gas species in the gas mixture. The method includes receiving output signals from two such sensors, processing the output signals in a controller that implements a model of the sensors so as to identify a fault in the first gas sensor or the second gas sensor; and providing an indication of any identified faults.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The relevant background includes the fields of exhaust gas treatment systems and diagnostics therefore. As to the former field of endeavor, there have been a variety of exhaust gas treatment systems developed in the art to minimize emission of undesirable constituent components of engine exhaust gas. It is known to reduce NOx emissions using a selective catalytic reduction (SCR) system, a treatment device that includes a catalyst and a system that is operable to inject material such as ammonia (NH 3 ) into the exhaust gas feedstream ahead of the catalyst. The SCR catalyst is constructed so as to promote the reduction of NOx by NH 3  (or other reductant, such as aqueous urea which undergoes decomposition in the exhaust to produce NH 3 ). NH 3  or urea selectively combine with NOx to form N 2  and H 2 O in the presence of the SCR catalyst, as described generally in U.S. Patent Application Publication 2007/0271908 entitled “ENGINE EXHAUST EMISSION CONTROL SYSTEM PROVIDING ON-BOARD AMMONIA GENERATION”, the contents of which are incorporated by reference. For diesel engines, for example, selective catalytic reduction (SCR) of NOx with ammonia is perhaps the most selective and active reaction for the removal of NOx in the presence of excess oxygen. The NH 3  source must be periodically replenished and the injection of NH 3  into the SCR catalyst requires precise control. Overinjection may cause a release of NH 3  (“slip”) out of the tailpipe into the atmosphere, while underinjection may result in inadequate emissions reduction (i.e., inadequate NOx conversion to N 2  and H 2 O). 
         [0002]    These systems have been amply demonstrated in the stationary catalytic applications. For mobile applications where it is generally not possible (or at least not desirable) to use ammonia directly, urea-water solutions have been proven to be suitable sources of ammonia in the exhaust gas stream. This has made SCR possible for a wide range of vehicle applications. 
         [0003]    Increasingly stringent demands for low tail pipe emissions of NOx have been placed on heavy duty diesel powered vehicles. Liquid urea dosing systems with selective catalytic NOx reduction (SCR) technologies have been developed in the art that provide potentially viable solutions for meeting current and future diesel NOx emission standards around the world. Ammonia emissions may also be set by regulation or simply as a matter of quality. For example, proposed future European emission standards (e.g., EU 6) for NH 3  slip targets specify 10 ppm average and 30 ppm peak. However, the challenge described above remains, namely, that such treatment systems achieve maximum NOx reduction (i.e., at least meeting NOx emissions criteria) while at the same time maintaining acceptable NH 3  emissions, particularly over the service life of the treatment system. 
         [0004]    In addition to the substantive emissions standards described above, vehicle-based engine and emission systems typically also require various self-monitoring diagnostics to ensure tailpipe emissions compliance. In this regards, U.S. federal and state on-board diagnostic regulations (e.g., OBDII) require that certain emission-related systems on the vehicle be monitored, and that a vehicle operator be notified if the system is not functioning in a predetermined manner. Automotive vehicle electronics therefore typically include a programmed diagnostic data manager or the like service configured to receive reports from diagnostic algorithms/circuits concerning the operational status of various components or systems and to set/reset various standardized diagnostic trouble codes (DTC) and/or otherwise generate an alert (e.g., MIL). The intent of such diagnostics is to inform the operator when performance of a component and/or system has degraded to a level where emissions performance may be affected and to provide information (e.g., via the DTC) to facilitate remediation. 
         [0005]    Over the service life of the above-described exhaust treatment systems, various constituent components can wear, degrade or the like, possibly impairing overall performance. For example, degradation of either the SCR catalyst or the dosing system may impair the treatment system in meeting either or both of the NOx and NH 3  emission standards. Diagnostic methods to detect such conditions are described generally in U.S. Patent Application Publication 2010/0101214 entitled “DIAGNOSTIC METHODS FOR SELECTIVE CATALYTIC REDUCTION (SCR) EXHAUST TREATMENT SYSTEMS”, the contents of which are incorporated by reference. However, improvements are always desirable in any art. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    In a first aspect of the invention, a method is presented for fault identification of gas sensors. The method includes receiving a first output signal from a first gas sensor having an output that varies according to both the concentration of a first gas species in a gas mixture and a second gas species in the gas mixture. The method further includes receiving a second output signal from a second gas sensor having an output that varies according to both the concentration of the first gas species in a gas mixture and the second gas species in the gas mixture. The method further includes processing the first output signal and the second output signal in a diagnostic controller that implements a model of the first gas sensor and a model of the second gas sensor so as to identify a fault in the first gas sensor or the second gas sensor. 
         [0007]    In a second aspect of the invention, a fault identification system for gas sensors includes a first gas sensor having an output that varies according to both the concentration of a first gas species in a gas mixture and a second gas species in the gas mixture. The system further includes a second gas sensor having an output that varies according to both the concentration of the first gas species in a gas mixture and the second gas species in the gas mixture. The system further includes a diagnostic controller that implements a model of the first gas sensor and a model of the second gas sensor so as to identify a fault in the first gas sensor or the second gas sensor. 
         [0008]    Further aspects of the invention will become apparent from the detailed description provided hereafter. It is to be understood that the detailed description and examples provided are intended for purposes of illustration and are not intended to limit the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a diagrammatic and block diagram showing an exhaust treatment system in which the diagnostic methods of the invention may be practiced. 
           [0010]      FIG. 2  is a graphical representation of the voltage across an NH 3  cell, the voltage across a NO X  cell, and the voltage across an NH 3 —NO X  cell, at selected partial pressures of NO X  and of NH 3  in a sample gas. 
           [0011]      FIG. 3  is a simplified electrical schematic depicting an interface between a gas sensor and an electrical apparatus. 
           [0012]      FIG. 4  is a flow chart of a first diagnostic method incorporating aspects of the present invention. 
           [0013]      FIG. 5  is a flow chart of a second diagnostic method incorporating aspects of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    Referring now to the drawings wherein like reference numerals are used to identify identical components in the various views,  FIG. 1  is a diagrammatic and block diagram showing an exemplary diesel cycle internal combustion engine  10  whose combustion exhaust gas  12  is fed to an exhaust gas treatment system  14 . The exhaust gas is represented as a stream flowing through the exhaust gas treatment system  14  and is shown as a series of arrows designated  12   EO  (engine out),  12   1 ,  12   2 ,  12   3  and  12   TP  (tail pipe). It should be understood that while the invention will be described in connection with an automotive vehicle (i.e., mobile) embodiment, the invention may find useful application in stationary applications as well. In addition, embodiments of the invention may be used in heavy-duty applications (e.g., highway tractors, trucks and the like) as well as light-duty applications (e.g., passenger cars). Moreover, embodiments of the invention may find further useful application in various types of internal combustion engines, such as compression-ignition (e.g., diesel) engines as well as spark-ignition engines. 
         [0015]    In the illustrative embodiment, the engine  10  may be a turbocharged diesel engine. In a constructed embodiment, the engine  10  comprised a conventional 6.6-liter, 8-cylinder turbocharged diesel engine commercially available under the DuraMax trade designation. It should be understood this is exemplary only. 
         [0016]      FIG. 1  also shows an engine control unit (ECU)  16  configured to control the operation of the engine  10 . The ECU  16  may comprise conventional apparatus known generally in the art for such purpose. Generally, the ECU  16  may include at least one microprocessor or other processing unit, associated memory devices such as read only memory (ROM) and random access memory (RAM), a timing clock, input devices for monitoring input from external analog and digital devices and controlling output devices. The ECU  16  is operable to monitor engine operating conditions and other inputs (e.g., operator inputs) using the plurality of sensors and input mechanisms, and control engine operations with the plurality of output systems and actuators, using pre-established algorithms and calibrations that integrate information from monitored conditions and inputs. It should be understood that many of the conventional sensors employed in an engine system have been omitted for clarity. The ECU  16  may be configured to calculate an exhaust mass air flow (MAF) parameter  20  indicative of the mass air flow exiting engine  10 . 
         [0017]    The software algorithms and calibrations which are executed in the ECU  16  may generally comprise conventional strategies known to those of ordinary skill in the art. Overall, in response to the various inputs, the ECU  16  develops the necessary outputs to control the fueling (fuel injector opening, duration and closing) and other aspects of engine operation, all as known in the art. 
         [0018]    In addition to the control of the engine  10 , the ECU  16  is also typically configured to perform various diagnostics. For this purpose, the ECU  16  may be configured to include a diagnostic data manager or the like, a higher level service arranged to manage the reports received from various lower level diagnostic routines/circuits, and set or reset diagnostic trouble code(s)/service codes, as well as activate or extinguish various alerts, all as known generally in the art. For example only, such a diagnostic data manager may be pre-configured such that certain non-continuous monitoring diagnostics require that such diagnostic fail twice before a diagnostic trouble code (DTC) is set and a malfunction indicator lamp (MIL) is illuminated. As shown in  FIG. 1 , the ECU  16  may be configured to set a corresponding diagnostic trouble code (DTC)  24  and/or generate an operator alert, such an illumination of a MIL  26 . Although not shown, in one embodiment, the ECU  16  may be configured so as to allow interrogation (e.g., by a skilled technician) for retrieval of such set DTCs. Generally, the process of storing diagnostic trouble codes and subsequent interrogation and retrieval is well known to one skilled in the art and will not be described in any further detailed. 
         [0019]    With continued reference to  FIG. 1 , the exhaust gas treatment system  14  may include a diesel oxidation catalyst (DOC)  28 , a diesel particulate filter (DPF)  30 , a dosing subsystem  32  including at least (i) a reductant (e.g., urea-water solution) storage tank  34  and (ii) a dosing unit  36 , and a selective catalytic reduction (SCR) catalyst  38 . In addition,  FIG. 1  shows various sensors disposed in and/or used by the treatment system  14 . These include a DOC inlet temperature sensor  39  configured to generate a DOC inlet temperature signal  41  (T DOC-IN ), a NOx sensor  40  configured to generate a NOx signal  42  (NOx) indicative of a sensed NOx concentration, a first exhaust gas temperature sensor  44 , located at the inlet of the SCR catalyst  38 , configured to generate a first temperature signal  46  (T IN ), an optional second exhaust gas temperature sensor  48  configured to generate a second temperature signal  50  (T OUT ), a first pressure sensor  52  configured to generate a first pressure signal  54  (P IN ), a second pressure sensor  56  configured to generate a second pressure signal  58  (P OUT ), and an ammonia (NH 3 ) concentration sensor  60  configured to generate an ammonia concentration signal  62  indicative of the sensed NH 3  concentration. In many commercial vehicles, a NOx sensor  64  is provided for generating a second NOx signal  66  indicative of the NOx concentration exiting the tail pipe. However, such is shown for completeness only. 
         [0020]    The DOC  28  and the DPF  30  may comprise conventional components to perform their known functions. 
         [0021]    The dosing subsystem  32  is responsive to an NH 3  Request signal produced by a dosing control  80  and configured to deliver a NOx reducing agent at an injection node  68 , which is introduced in the exhaust gas stream in accurate, controlled doses  70  (e.g., mass per unit time). The reducing agent (“reductant”) may be, in general, (1) NH 3  gas or (2) a urea-water solution containing a predetermined known concentration of urea. The dosing unit  32  is shown in block form for clarity and may comprise a number of sub-parts, including but not limited to a fluid delivery mechanism, which may include an integral pump or other source of pressurized transport of the urea-water solution from the storage tank, a fluid regulation mechanism, such as an electronically controlled injector, nozzle or the like (at node  68 ), and a programmed dosing control unit. The dosing subsystem  32  may take various forms known in the art and may comprise commercially available components. 
         [0022]    The SCR catalyst  38  is configured to provide a mechanism to promote a selective reduction reaction between NOx, on the one hand, and a reductant such as ammonia gas NH 3  (or aqueous urea, which decomposes into ammonia, NH 3 ) on the other hand. The result of such a selective reduction is, as described above in the Background, N 2  and H 2 O. In general, the chemistry involved is well documented in the literature, well understood to those of ordinary skill in the art, and thus will not be elaborated upon in any greater detail. In one embodiment, the SCR catalyst  38  may comprise copper zeolite (Cu-zeolite) material, although other materials are known. See, for example, U.S. Pat. No. 6,576,587 entitled “HIGH SURFACE AREA LEAN NOx CATALYST” issued to Labarge et al., and U.S. Pat. No. 7,240,484 entitled “EXHAUST TREATMENT SYSTEMS AND METHODS FOR USING THE SAME” issued to Li et al., both owned by the common assignee of the present invention, and both hereby incorporated by reference in their entirety. In addition, as shown, the SCR catalyst  38  may be of multi-brick construction, including a plurality of individual bricks  38   1 ,  38   2  wherein each “brick” may be substantially disc-shaped. The “bricks” may be housed in a suitable enclosure, as known. 
         [0023]    The NOx concentration sensor  40  is located upstream of the injection node  68 . The NOx sensor  40  is so located so as to avoid possible interference in the NOx sensing function due to the presence of NH 3  gas. The NOx sensor  40 , however, may alternatively be located further upstream, between the DOC  28  and the DPF  30 , or upstream of the DOC  28 . In addition, the exhaust temperature is often referred to herein, and for such purpose, the temperature reading from the SCR inlet temperature sensor  44  (T IN ) may be used. 
         [0024]    The NH 3  sensor  60  may be located, in certain embodiments, at a mid-brick position, as shown in solid line (i.e., located anywhere downstream of the inlet of the SCR catalyst  38  and upstream of the outlet of the SCR catalyst  38 ). As illustrated, the NH 3  sensor  60  may be located at approximately the center position. The mid-brick positioning is significant. The sensed ammonia concentration level in this arrangement, even during nominal operation, is at a small yet detectable level of mid-brick NH 3  slip, where the downstream NOx conversion with this detectable NH 3  can be assumed in the presence of the rear brick, even further reducing NH 3  concentration levels at the tail pipe to within acceptable levels. Alternatively, in certain embodiments, the NH 3  sensor  60  may be located at the outlet of the SCR catalyst  38 . The remainder of the sensors shown in  FIG. 1  may comprise conventional components and be configured to perform in a conventional manner known to those of ordinary skill in the art. 
         [0025]    The dosing control  80  is configured to generate the NH 3  Request signal that is sent to the dosing unit  36 , which represents the command for a specified amount (e.g., mass rate) of reductant to be delivered to the exhaust gas stream. The dosing control  80  includes a plurality of inputs and outputs, designated  18 , for interface with various sensors, other control units, etc., as described herein. Although the dosing control  80  is shown as a separate block, it should be understood that depending on the particular arrangement, the functionality of (the dosing control  80  may be implemented in a separate controller, incorporated into the ECU  16 , or incorporated, in whole or in part, in other control units already existing in the system (e.g., the dosing unit). Further, the dosing control  80  may be configured to perform not only control functions described herein but perform the various diagnostics also described herein as well. For such purpose, the dosing control  80  may include conventional processing apparatus known in the art, capable of executing pre-programmed instructions stored in an associated memory, all performing in accordance with the functionality described herein. That is, it is contemplated that the control and diagnostic processes described herein will be programmed in a preferred embodiment, with the resulting software code being stored in the associated memory. Implementation of the invention, in software, in view of the foregoing enabling description, would require no more than routine application of programming skills by one of ordinary skill in the art. Such a control may further be of the type having both ROM and RAM, a combination of non-volatile and volatile (modifiable) memory so that the software can be stored and yet allow storage and processing of dynamically produced data and/or signals. 
         [0026]    The ammonia (NH 3 ) concentration sensor  60  may comprise a gas sensor as described generally in U.S. Patent Application Publication 2010/0032292 entitled “AMMONIA GAS SENSOR”, the contents of which are incorporated by reference. This sensor includes a first electrode material that is sensitive to an NH 3  concentration in the sensed gas but which is also vulnerable to cross-interference from NO 2  concentration in the sensed gas. A second electrode material is also provided that has an electrochemical sensitivity to NO 2  that is greater than its sensitivity to NH 3  or NO. The disclosure of Patent Application Publication 2010/0032292 details how signals from the two sensor electrode materials can be processed to provide an improved determination of NH 3  concentration. While the details of this disclosure will not be repeated here, it is useful to discuss the characteristics of the disclosed sensor electrode materials as an aid to appreciating aspects of the present invention. 
         [0027]    Referring to  FIG. 2 , a graphical representation  100  of the voltage outputs of a gas sensor is shown. The tested sensor had a BiVO 4  (5% MgO) NH 3  electrode, a TbMg 0.2 Cr 0.8 O 3 NO x  electrode, and a Pt reference electrode. The sensor was operated at 560° C. The graphical representation includes a line representing the voltage (line  102 ) across the NH 3  sensing cell, a line representing the voltage (line  104 ) across the NO x  sensing cell, and a line  106  representing the voltage across the NH 3 —NO x  cell. The graphical representation  100  further includes four intervals representing NO 2  and NO concentrations: a first interval  108  where NO and NO 2  concentrations are 0 ppm (parts per million), a second interval  110  where NO concentration is 400 ppm and NO 2  concentration is 0 ppm, a third interval  112  where NO concentration is 200 ppm and NO 2  concentration is 200 ppm, and a fourth interval  114  where NO concentration is 0 ppm and NO 2  concentration is 400 ppm. 
         [0028]    Each of the intervals  108 ,  110 ,  112 ,  114 , include seven subsections representing NH 3  concentrations: a first subsection  116  where the NH 3  concentration is 100 ppm, a second subsection  118  where the NH 3  concentration is 50 ppm, a third subsection  120  where the NH 3  concentration is 25 ppm, a fourth subsection  122  where the NH 3  concentration is 10 ppm, a fifth subsection  124  where the NH 3  concentration is 5 ppm, a sixth subjection  126  where the NH 3  concentration is 2.5 ppm, and a seventh subjection  128  where the NH 3  concentration is 0 ppm. The remaining gas is composed of 10% O 2 , 1.5% of H 2 O and balanced by N 2 . 
         [0029]    As shown in  FIG. 2 , the line  102  representing the voltage across the NH 3  sensing cell is identical in intervals  108  and  110  where NO 2  is excluded from the gas being measured. However, the voltage across the NH 3  sensing cell represented by line  102  has a lower value (higher absolute value) in section  112  and  114  where NO 2  is present, thereby demonstrating the cross-interference effect of NO 2  on the NH 3  sensing cell. 
         [0030]    Similarly,  FIG. 2  also shows the cross-interference effect of NH 3  on the NO 2  sensing cell. Within any of the intervals  110 ,  112 ,  114  where NOx is present, the line  104  representing the voltage across the NOx sensing cell shows the influence of NH 3  concentration as the NH 3  concentration is varied from 100 ppm in subsection  116  of each interval to 0 ppm in subsection  128  of each interval. 
         [0031]    The system and method disclosed herein take advantage of these mutual cross-interference effects to enable improved fault determination of the sensors. In an aspect of the system and method of the invention, the output signals produced by each of the two electrode materials are compared to determine if the effects produced by concentrations of NH 3  and NO 2  are consistent with the known cross-interference characteristics of the electrode materials. These aspects will be further described by way examples to follow. 
         [0032]      FIG. 3  is a schematic diagram that depicts how the sensors may be connected in a system. In  FIG. 3 , a sensor assembly is depicted generally as  160 , with the sensor assembly  160  including a first sensing cell  162 , a second sensing cell  166 , and a heater  170  thermally coupled to both sensing cells  162  and  166 . The sensing cell  162  produces a voltage EMF 1  that is related to the concentrations of gas species, and the sensing cell  166  produces a voltage EMF 2  that is related to the concentrations of gas species. The sensing cells  162  and  166  may be considered to include associated source impedances  164  and  168  respectively. Both the emf EMF 1 , EMF 2  and the source impedance  164 ,  168  of a sensing cell  162 ,  166  are influenced by the temperature of the sensing cell, and the heater  170  is controlled to maintain the temperature of the sensing cell  162 ,  166  at a desired level. The sensor  160  may also include a temperature sensor (not shown) to sense a temperature produce by the heater  170 . It will be appreciated that, while  FIG. 3  shows two emf cells in thermal communication with a single heater, the emf cells may be contained in separate physical embodiments, and each emf cell may have its own associated heater. 
         [0033]    Still referring to  FIG. 3 , the sensor  160  is electrically connected to an interface apparatus generally depicted as  180 . Apparatus  180  is depicted as including a measurement means  186  to measure the voltages produced by the sensing cells  162 ,  166 . Apparatus  180  also includes a heater control means in electrical communication with the heater  170  to maintain the heater  170  at a desired temperature. Additionally, apparatus  180  is shown as containing a first pull-up resistor  182  connected from the output of the first sensing cell  162  to a voltage source V+, and a second pull-up resistor  184  connected from the output of the second sensing cell  166  to a voltage source V+. Reference will be made to  FIG. 3  in the discussion of the following examples. 
       EXAMPLE 1 
     Sensor Rationality Test during Reductant Dosing 
       [0034]    A first diagnostic method may be used during intervals when reductant is being added to the exhaust gas, e.g. when a urea solution is being injected. During such a time interval, the gas to which the exhaust sensor is exposed will have a relatively high concentration of NH 3 . As illustrated in  FIG. 2 , both the NH 3  sensing cell (whose output is shown in trace  102 ) and the NOx sensing cell (whose output is shown in trace  104 ) are influenced by the concentration of NH 3  in the sensed gas. In the discussion that follows, the output of the NH 3  sensing cell will be denoted as EMF 1 , and the output of the NOx sensing cell will be denoted as EMF 2 . 
         [0035]    Referring to  FIG. 4 , the first diagnostic method  200  includes the step  205  of receiving EMF 1  and EMF 2  values from the NH 3  sensing cell and the NOx sensing cell respectively. In decision step  210 , the measured values of EMF 1  and EMF 2  received in step  205  are each compared to a predetermined range for the respective sensor. As will be appreciated from  FIG. 3 , a value of EMF 1  measured during reductant dosing that is too low may be an indication of a short circuit across or other damage to sensing cell  162 . A value of EMF 2  measured during reductant dosing that is too low may be an indication of a short circuit across or other damage to sensing cell  166 . A measured value of EMF 1  or EMF 2  that is too high may be the result of a high impedance in sensing cell  162  or  166  as may be caused by a damaged sensor or improper operation of heater  170  or heater control  188 . A high measured value of EMF 1  may also be the result of an open conductor or connector in the circuit between measurement means  186  and sensing cell  162 , resulting in measurement means  186  receiving V+ through pull-up resistor  182 . A high measured value of EMF 2  may also be the result of an open conductor or connector in the circuit between measurement means  186  and sensing cell  166 , resulting in measurement means  186  receiving V+ through pull-up resistor  184 . If EMF 1  and/or EMF 2  stay at a constant value in excess of a predetermined amount of time, this may be an indication that the sensing cells  162  and/or  166  may be isolated from the exhaust gas, for example because of a sensor shield or coating layer being plugged by soot or by chemicals that are poisonous to the sensor. If the result of decision step  220  is that the measured value of EMF 1  and/or EMF 2  is outside a predetermined range, the process flow proceeds to step  240 . 
         [0036]    In step  215 , the concentration of NH 3  is determined from the received values of EMF 1  and EMF 2 . The concentration of NH 3  may be determined based on a calculation involving a predetermined characteristic equation relating EMF 1  and EMF 2  to NH 3  concentration. An exemplary characteristic equation is disclosed in U.S. patent application Ser. No. 12/974,266 titled “METHOD AND DEVICE FOR CHARACTERIZATION AND SENSING OF EXHAUST GAS AND CONTROL OF ENGINES AND COMPONENTS FOR AFTERTREATMENT OF EXHAUST GASES” filed Dec. 21, 2010, the contents of which are hereby incorporated by reference. Alternatively, the concentration of NH 3  may be determined in step  215  by means of a lookup table that uses EMF 1  and EMF 2  as inputs. 
         [0037]    In step  220 , the concentration of NH 3  that was determined in step  215  is used to determine a predicted EMF 2  value, based on a predetermined relationship describing the sensitivity of the NOx sensing cell to NH 3  concentration. The predicted value of EMF 2  may be determined based on a calculation based on a predetermined mathematical model for the NOx sensing cell that relates EMF 2  to NH 3  concentration. Alternatively, the predicted value of EMF 2  may be determined by means of a table look-up using NH 3  concentration as an input. 
         [0038]    Still referring to  FIG. 4 , the method includes a further sequence of decision steps  225 ,  230 , and  235 . Step  225  compares the predicted value of EMF 2  based on the NH 3  concentration to a predetermined range. If the predicted value of EMF 2  is outside of the predetermined range, this is indicative of degradation of one or both of sensing cells  162 ,  166 , such as may result from cell aging or poisoning. If the result of decision step  225  is that the predicted value of EMF 2  is outside a predetermined range, the process flow proceeds to step  240 . 
         [0039]    If the test in step  225  does not indicate a fault condition, the method continues to step  230 . In this step, the difference (predicted value of EMF 2 —measured value of EMF 2 ) is compared to a predetermined threshold. If this difference is below the threshold, this may be an indication of a malfunction in the NH 3  sensing cell, resulting in an underestimation of NH 3  concentration in step  210  and a corresponding underestimation of the predicted value of EMF 2  in step  215 . If a malfunction is indicated, the method proceeds to step  240 . 
         [0040]    If the test in step  230  does not indicate a fault condition, the method continues to step  235 . In this step, the difference (predicted value of EMF 2 —measured value of EMF 2 ) is compared to a predetermined threshold. If the difference is above this threshold, this may be an indication of a malfunction in the NOx sensing cell, resulting in the cell not exhibiting the cross-influence effect to NH 3  that is known to be a characteristic of the NOx sensing cell. If a malfunction is indicated by step  230 , the method proceeds to step  240 . If no malfunction is detected, the diagnostic routine  200  is exited. 
         [0041]    Step  240  in method  200  is entered upon detection of a fault condition by any of the decision steps  220 ,  225 ,  230 , or  235 . Step  240  indicates the appropriate fault condition. The response of the system to a fault condition may depend on the nature of the fault condition. For example, a diagnostic trouble code (DTC) may be set and/or a malfunction indicator lamp (MIL) may be illuminated. Depending on the nature of the fault condition, control of the engine or exhaust treatment systems may be changed to a failsafe backup mode to preserve driveability and/or to prevent damage to other components. 
       EXAMPLE 2 
     Sensor Rationality Test during Intervals of No Reductant Dosing 
       [0042]    A second diagnostic method may be executed during times when no reductant is being added to the exhaust gas. During such a time interval, the gas to which the exhaust sensor is exposed will contain a substantial quantity of NO 2  which may be predetermined by engine mapping or by direct measurement, and will contain essentially zero NH 3 . Again, in the discussion that follows, the output of the NH 3  sensing cell will be denoted as EMF 1 , and the output of the NOx sensing cell will be denoted as EMF 2 . 
         [0043]    Referring again to  FIG. 2 , it will be appreciated that under conditions of negligible NH 3  (as seen in subsections  128  in intervals  108 ,  110 ,  112 , and  114 ), EMF 1  (shown as line  102 ) shows appreciable sensitivity to NO 2  concentration. Recall that intervals  108  and  110  represent conditions in which NO 2  is excluded from the gas being measured, interval  112  represents 200 ppm NO 2 , and interval  114  represents 400 ppm NO 2 . An aspect of the present invention takes advantage of this cross-interference effect of NO 2  on the NH 3  sensing cell at low NH 3  levels to provide additional diagnostic information. 
         [0044]    Referring to  FIG. 5 , the second diagnostic method  300  includes the step  305  of receiving EMF 1  and EMF 2  values from the NH 3  sensing cell and the NOx sensing cell respectively. 
         [0045]    In step  315 , predicted values of EMF 1  and EMF 2  are determined. The determination of predicted values of EMF 1  and EMF 2  may be based on predetermined engine mapping information relating the levels of NO and NO 2  in the exhaust to the engine operating conditions. Alternately, predicted values of EMF 1  and EMF 2  may be determined based on measured NOx levels from another sensor such as, for example, sensor  40  in  FIG. 1 . The determination of predicted values of EMF 1  and EMF 2  may further be based on predetermined sensor characterization relating EMF 1  and EMF 2  to levels of NO and NO 2  in the exhaust. The determination of predicted values of EMF 1  and EMF 2  may be accomplished by means of look-up tables, calculations utilizing equations, or a combination thereof. Alternatively, the measured values of EMF 1  and EMF 2  may be utilized to calculate NO and NO 2  (NOx) based on a predetermined sensor model. The calculated NO and NO 2  (NOx) may be compared with predicted values of NO and NO 2  NOX) based on predetermined engine mapping, or with another NOx sensor such as sensor  40 . 
         [0046]    Step  320  in method  300  compares the predicted value of EMF 1  determined in step  315  to the measured value of EMF 1  received in step  305 . If the difference between the predicted and measured values is outside of a predetermined range, this is indicative of a fault condition. For example, a measured value of EMF 1  that is significantly less than the predicted value of EMF 1  may indicate a short circuit across or other damage to sensing cell  162 . A lower than predicted value of EMF 1  may also result from thermal damage (meltdown) or chemical poisoning of sensing cell  162 . 
         [0047]    A measured value of EMF 1  that is significantly greater than the predicted value of EMF 1  may be indicative of a high impedance in sensing cell  162  as may be caused by a damaged sensing cell  162  or improper operation of heater  170  or heater control  188 . A higher than predicted value of EMF 1  may also be the result of an open conductor or connector in the circuit between measurement means  186  and sensing cell  162 , resulting in measurement means  186  receiving V+ through pull-up resistor  182 . If the result of decision step  320  is that the measured value of EMF 1  differs from the predicted value of EMF 1  in excess of a predetermined amount, the process flow proceeds to step  340 . 
         [0048]    If the test of EMF 1  in step  320  does not indicate a fault condition, step  325  performs a similar test on EMF 2  by comparing the predicted value of EMF 2  determined in step  315  to the measured value of EMF 2  received in step  305 . If the difference between the predicted and measured values is outside of a predetermined range, this is indicative of a fault condition. For example, a measured value of EMF 2  that is significantly less than the predicted value of EMF 2  may indicate a short circuit across or other damage to sensing cell  166 . A lower than predicted value of EMF 2  may also result from thermal damage (meltdown) or chemical poisoning of sensing cell  166 . 
         [0049]    A measured value of EMF 2  that is significantly greater than the predicted value of EMF 2  may be indicative of a high impedance in sensing cell  166  as may be caused by a damaged sensing cell  166  or improper operation of heater  170  or heater control  188 . A higher than predicted value of EMF 2  may also be the result of an open conductor or connector in the circuit between measurement means  186  and sensing cell  166 , resulting in measurement means  186  receiving V+ through pull-up resistor  184 . If the result of decision step  325  is that the measured value of EMF 1  differs from the predicted value of EMF 1  in excess of a predetermined amount, the process flow proceeds to step  340 . If no malfunction is detected, the diagnostic routine  300  is exited 
         [0050]    Step  340  in method  300  is entered upon detection of a fault condition by either of the decision steps  320  or  325 . Step  340  indicates the appropriate fault condition. The response of the system to a fault condition may depend on the nature of the fault condition. For example, a diagnostic trouble code (DTC) may be set and/or a malfunction indicator lamp (MIL) may be illuminated. Depending on the nature of the fault condition, control of the engine or exhaust treatment systems may be changed to a failsafe backup mode to preserve driveability and/or to prevent damage to other components. 
         [0051]    In the foregoing examples, the indicated orders of the steps of the method are for illustration purposes only. One skilled in the art will appreciate that certain steps may be performed in different orders without departing from the inventive concepts disclosed herein. While this invention has been described in terms of the embodiments thereof, it is not intended to be so limited, but rather only to the extent set forth in the claims that follow.