UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

ANN ARBOR, MICHIGAN 48105 

OFFICE OF 

AIR AND RADIATION 

MEMORANDUM 

DATE: March 07,2013 

SUBJECT: "Best Practices for Addressing OBD Readiness in IM Testing of Diesel Vehicles 

Under 14,000 Pounds Gross Vehicle Weight Rating" 

FROM: Leila Holmes Cook, Director 

State Measures and Transportation Planning Center 

Transportation and Climate Division 

TO: Vehicle Inspection and Maintenance (IM) Program Managers and Technical 

Workgroup Participants 

This memo transmits the above-named "Best Practices for Addressing On-Board Diagnostic 

(OBD) Readiness in IM Testing ofDiesel Vehicles Under 14,001 Pounds (lbs.) Gross Vehicle 

Weight Rating (GVWR)" which was prepared primarily by Eastern Research Group (ERG) 

under contract to U.S. EPA with input from a group of outside experts in the field of on board 

diagnostics (OBD) testing issues as they apply to light- to medium-duty diesel vehicles. In 

providing this technical forum EPA was responding to requests for program implementation 

advice from state IM programs interested in pursuing OBD testing of light- to medium-duty 

diesel vehicles. While testing these vehicles is not mandatory, EPA was also responding to 

concerns expressed by diesel vehic le manufacturers and their representatives that testing such 

vehicles using EPA's guidance for OBD testing of gasoline-fueled vehicles could lead to high 

rejection rates of vehicles for having too many unset readiness monitors. The attached document 

reflects the thinking of the workgroup participants regarding the best way to minimize motorist 

inconvenience due to readiness-related issues when testing these vehicles. This document is 

provided as a courtesy by EPA to interested states to help those who choose to test these 

vehicles, but should not be construed as an endorsement of such testing, nor as official EPA 

guidance on the matter. 
Best Practices for Add ressing On-Board Diagnostic (OBD) Readiness in IM Testing of 

Diesel Vehicles Under 14,001 Pounds (lbs.) Gross Vehicle Weight Rating (GVWR) 

Background 

Although not required by EPA regulation, a number of state inspection and maintenance (IM) 

programs are currently performing on-board diagnostic (OBD) tests on model year 1997 (MY 

1997) and newer OBD certified light-duty (LD) and medium-duty (MD) diesel vehicles. Some 

reasons cited by states for testing these vehicles include the following: 

• Overall equity, because MY 1996 and newer gasoline vehicles equipped with the same 

OBD technology are required to be tested in areas which operate IM programs 

• From a technical standpoint, it seems logical to include these vehicles in the IM program 

if indeed they are equipped with OBD technology. 

• Although exhaust from newer diesel vehicles contains much less smoke than in the past, 

it can still be visible at times, and this in tum can lead to the public raising the question as 

to why these vehicles are not required to be tested. 

• Finally, ifOBD testing ofthese vehicles proves viable and the emission benefits can be 

quantified, there is a possibility that those states with challenging air quality goals will 

look to include these benefits in future air quality planning projections. However, the 

issue of credit in state implementation plans is not part of this workgroup's charge . 

• Issue Statement: OBD readiness status has been identified as an important component when IM 

testing a gasoline-fueled vehicle because it helps ensure that the vehicle 's emission system is 

performing properly and also helps in detecting attempts to fraudulently pass an 1M test. 

Similarly, OBD readiness must be addressed when conducting OBD tests on LD/MD diesel 

vehicles. However, diesel emission control technology and monitoring strategies differ 

significantly compared to gasoline vehicles and, therefore, it is not a given that readiness can be 

treated the same way for these vehicles as for their gasoline-fueled counterparts. 

Specifically, some diesel OBD monitors, such as the particulate matter (PM) filter monitor, can 

take much more time to set readiness than comparable gasoline monitors. A gasoline OBD 

system may typically run all monitors within two weeks, while a diesel OBD system can take 

eight weeks or more to run all monitors. This is because certain diesel monitors are often linked 

to PM trap regeneration, and multiple regeneration events may be required to establish readiness. 

These monitors may include major emission controls such as the PM filter, the NMHC (nonmethane hydrocarbon) catalyst, the oxides of nitrogen (NOx) and selective catalytic reduction 

(SCR) aftertreatment devices and, in some cases, exhaust sensors. Therefore, using the same 

readiness criteria (i.e., how many monitors can be "not ready" without the vehicle being failed or 

rejected) as provided for gasoline vehicles may result in an intolerable number of readiness 

failures for LD and MD diesel vehicles. The diesel readiness criteria recommended by the 

workgroup to address this issue is described in the sections below. 

Formation of Workgroup: EPA identified recognized experts in the field of OBD diesel 

technology. EPA then tasked Eastern Research Group, Inc. (ERG) 1 to convene a workgroup 

1 Eastern Research Group, Inc. (ERG) EPA contract EP-C-11-046, Work Assignments 0-10 and 1-10. 
comprised of those recognized experts. The purpose of the workgroup was to develop a best 

practices document (BPD) to address OBD readiness issues for those programs wishing to 

perform OBD tests on diesel vehicles less than or equal to 14,000 lbs. GVWR. The members of 

the workgroup are listed in Appendix A. 

Scope: The scope of this document is limited to MY 1997+ OBD-equipped diesel vehicles less 

than or equal to 14,000 lbs. GVWR. This GVWR range covers LD diesel (~8 500 GVWR) and 

MD diesel (>8,500 and 14,000 GVWR). 

Technical Information 

There are a number of basic requirements covering various vehicle parameters and test 

procedures that are already being used for gasoline vehicle inspections that should also be 

followed in diesel OBD testing. This section provides an overview of typical major elements of 

an OBD inspection and, where relevant, a comparison between typical diesel and gasoline 

vehicles with respect to the element. This section is not meant to be an exhaustive list of all 

elements of an OBD inspection. 

I. Diagnostic Trouble Codes (DTCs): DTCs are used by the OBD system to identify 

malfunctioning components or systems. Generic fault codes2 

are used for both gasoline 

OBD systems and diesel OBD systems. Existing IM equipment can read codes from 

diesel vehicles in the same manner with which they are read from gasoline vehicles. 

2. Permanent Diagnostic Trouble Codes (Permanent-DTCs): Permanent-DTCs are an 

additional feature that was phased-in on the OBD systems of MY 20 I 0 through 20123 

gasoline and diesel vehicles. Because this BPD will include recommendations to use 

Permanent-DTCs when evaluating the readiness status of diesel vehicles so equipped, 

some background information on Permanent-DTCs is provided below. 

The primary difference between a Permanent-DTC and a regular DTC is that Permanent-DTCs 

are stored in memory and cannot be erased with scan tools (generic, aftermarket, or 

manufacturer-specific) or by disconnecting the battery, but instead can only be erased by the 

OBD system itself. When the Malfunction Indicator Light (MIL) is commanded "on" in a 

vehicle that supports Permanent-DTCs, the trouble code is stored as both a regular DTC and also 

as a Permanent-DTC. Unlike the regular DTC, the Permanent-DTC is written to non-volatile 

memory (NVRAM) at the end of the key cycle4

• Once stored, Permanent-DTCs can only be 

erased by the OBD system itself in one of two ways. Fir~t , if a scan tool "code clear" has not 

been received, the Permanent-DTC will be erased at the same time the MIL turns off(e.g., ifthe 

OBD system detects that a fault is no longer present on three consecutive trips, it will extinguish 

the MIL and erase the Permanent-DTC). Secondly, if a scan tool "code clear" has taken place 

2 SAE 1201 2 3 Note Title 13, Californ ia Code of Regulations, Section 1968.2 and 40 CFR Part 86.008-10 and 86.0 I 0-2. Beginning with MY 

20 10, Permanent-DTCs were phased-in. Fifty percent of the fl eet was required to be Permanent-DTC compliant in MY 2010; 

75% in MY 20 II , and I 00% in MY 2012. This requirement applied to both the gasoline and diesel fl eets. A scan tool complaint 

with SAE J 1979 version May 2007 or newer will be capable of automatically determining if a vehicle supports Permanent-DTCs. 4 NYRAM retains its in fo rmation when power is turned off. This is in contrast to dynamic random access memory and static 

random access memory which both retain data only for as long as power is applied. 

2 
(e.g., technician clears codes after repairing a fault), the Permanent-DTC will not be erased until 

the monitor responsible for setting that specific Permanent-DTC has run at least once and 

confirmed that the fault is no longer present. The system is designed to be able to store at least 

four Permanent-DTCs at one time. Should a vehicle have more than one DTC currently 

commanding the MIL on, all of the faults would be stored as Permanent-DTCs and could not be 

erased until the monitors responsible for each of the Permanent-DTCs has run and confirmed that 

each of the faults has been corrected. Permanent-DTCs are reported through a different scan tool 

command (11979 Mode/Service $0A) than regular DTCs, so inspection equipment/tools can 

distinguish between the two. 

Permanent-DTCs have been implemented to make it more difficult for a vehicle owner or 

technician to clear the fault memory, which can potentially allow the vehicle to pass inspection 

without being repaired. While readiness monitors were also originally intended to address this 

issue, Permanent-DTCs address many of the shortcomings that have led to the existing readiness 

policy. Permanent-DTCs also provide a much more direct and accurate indication that an 

attempt may have been made by a vehicle owner or technician to circumvent the inspection. 

Specifically, readiness monitors can be unset for multiple reasons, including those unrelated to a 

recent emission-related fault, while the presence of a Permanent-DTC provides a definitive 

indication that the car being inspected previously had the MIL on for that very fault and has not 

yet re-run that very same diagnostic to determine if the fault is still present. Like unset 

readiness, the presence of a Permanent-DTC does not definitively tell you ifthe vehicle still has 

a fault or not so it is appropriate to have such cars fail the readiness check until either the 

Permanent-DTC is erased (because the fault is gone) or the MIL is back on because the fault was 

re-detected. 

Given that Permanent-DTCs are phased in over the 2010 through 2012 model years and that 

diesel vehicles generally make up a small portion of a vehicle manufacturer's fleet, Table 1 was 

created to generally identify when most diesel vehicles had the feature implemented. However, 

per J 1979, scan tools and/or inspection equipment can be designed to automatically determine if 

a vehicle supports Permanent-DTCs. Use of a scan tool is the more accurate (and preferred) 

method for making this determination in an inspection program. 

Table 1: Manufacturer Permanent-DTC Compliance5 

Manufacturer MY with full Permanent-DTC Compliance 

BMW 2012 

Cummins 2010 

Ford 2012 

GM 2012 

Mercedes 2012 

vw 2012 

5 One hundred percent manufacturer compliance with Permanent-DTC reporting for all vehicles and engines began with the 

201 2MY the compliance deadline for the phase-in period. However, certified engines.are often installed in vehicles that are a 

model year newer. There fore, some 20 12 medium-duty vehicles may be equipped with 20 II model year engines that may not 

support Permanent-DTCs. All 2013 and later model year medium-duty vehicles should be equipped with engines that support 

Permanent-DTCs. 

3 
3. MY coverage: Newer diesel vehicles have been equipped with increasing numbers of 

emission controls because tailpipe emission standards have become more stringent. 

Accordingly, more diagnostics have been incorporated into OBD systems to monitor the 

various controls. While there are various milestones that have occurred over time with 

respect to what typical emission controls were implemented in what timeframe, the testing 

options described in this best practices document are designed to apply to all OBD 

equipped diesel vehicles. 

4. Communication protocols: Light duty and medium duty diesel vehicles use the same 

communication protocols that gasoline vehicles use. Therefore, the existing hardware 

and software currently in use for gasoline OBD IM testing should also be able to 

communicate with LD and MD diesel vehicles. As a point of reference, all MY 2008 and 

newer diesel vehicles with GVWRs :SI4,000 lbs. are equipped with the ISO 15765 

Controller Area Network (CAN) communication protocol. 

5. OBD readiness monitors: There are 11 readiness monitor bits in a diesel OBD system, 

which is the same number as in a gasoline OBD system. However, as can be seen in 

Table 2, only nine of the eleven bits are actually assigned to monitors at this point. Also, 

the monitor descriptions are different6 from comparable gasoline-fueled vehicles for MY 

20 I 0 and newer diesel vehicles, reflecting the fact that the different fuel systems use 

different emission controls. For example, a state's current vehicle inspection report (VIR) 

that identifies the 41

h monitor as the "Catalyst" in a gasoline vehicle should report the 

monitor as the "NMHC Catalyst" in a 20 I 0 or newer diesel vehicle (the readiness status of 

both are reported under SAE J1979 Parameter Identification (PID) $01 , Byte D, bit 0)7

• 

Per SAE J 1979, inspection equipment can be designed or updated to use information in 

the readiness data itself (PID $01 , Byte B, bit 3) to automatically determine whether the 

gasoline and pre-MY 20 I 0 diesel monitor descriptions or the MY 201 0+ diesel monitor 

descriptions apply to the vehicle. 

Table 2: Gasoline and Diesel OBD Readiness Monitors 

Monitor Readiness Monitor Assignments for all New Readiness Monitor Assignments 

Number Gasoline and for Diesel through MY 2009 for Diesel from MY 2010+ 

I Misfire Misfire 

2 Fuel System Fuel System 

3 Comprehensive Components Comprehensive Components 

4 Catalyst NMHC Catalyst 

5 Heated Catalyst NOx/SCR Aftertreatment 

6 Evaporative System (Reserved for future) 

7 Secondary Air Boost Pressure 

8 AIC (Reserved for future) 

9 02 Sensor Exhaust Gas Sensor 

10 Heated 02 Sensor PM Filter 

II EGRIVVT EGRIVVT 

6 SAE J 1979 PID$0 I ,Data Bytes B( bit #3), C, and D 7 For a comprehensive list of OBD PIDs, refer to SAE J 1979. 

4 
Also, in a gasoline OBD system, the first three monitors in Table 2 above are considered 

"continuous" and therefore are always reported as complete and/ or ready. As a result, many 

states do not include these three monitors as part of the lM readiness evaluation. However, for 

LD and MD diesels vehicles, only the last of the first three monitors- "comprehensive 

components" - is always reported as complete and/or ready. Accordingly, this BPD 

recommends that IM programs consider all assigned readiness monitors when evaluating the 

readiness status ofOBD-equipped, LD and MD diesel vehicles. 

6. Diagnostic Link Connector (DLC), Communications, and DTC Issues: The existing 

recommendations for gasoline OBD systems with regard to DLC issues such as 

"missing," "tampered," or "malfunctioning DLCs" should be followed for diesel OBD 

test programs as well. Likewise, "failure to communicate", pending DTCs, DTCs with 

no MIL illumination, and/or a MIL commanded on with no DTCs should all be handled 

in the same way as in the state's current gasoline IM program. 

7. Fraud Detection: Programs should use the same techniques for detecting "fraud" and 

"tampering" as in the state's current gasoline IM program. 

8. Individual Monitor Exemptiohs or Requirements: Although not required, many states 

have found it beneficial to design their inspection software to provide "readiness 

exceptions" for certain problematic vehicles. Options have included allowing a greater 

total number of"not ready" monitors (i .e., 2 instead of I) and the ability to " ignore" a 

specific monitor8 when making the IM readiness determination for a specific vehicle. 

These exceptions can prevent motorists from having their vehicles rejected for readiness 

when in reality there is nothing they can reasonably do to bring the vehicle into 

compliance with the base readiness criteria. 

Conversely, some IM programs for gasoline-fueled vehicles have required the catalyst 

monitor to be set to complete for the initial test (and for the re-inspection in cases where 

the vehicle was initially failed for a catalyst-based DTC). This requirement helps to 

further protect emission benefits by ensuring that the most important emission control has 

been monitored and passed at the time of the inspection. Other states have accepted 

proof of repair in lieu of meeting certain readiness criteria (for example, proof of a 

catalyst repair in lieu of the catalyst monitor being set to ready for there-inspection). 

These variations can further help to balance motorist inconvenience considerations with 

the need to ensure that critical emission controls are functioning properly and that 

components in need of repair have indeed been repaired. 

It is recommended that programs that have these types offlexibilities for gasoline 

vehicles also use them as needed for diesel vehicles, and states should consider including 

such capabilities in any future software revisions or upgrades. 

8 For example, the 1997 Toyota Tercel and Paseo were unable to set the Evaporative monitor. These vehicles were subsequently 

identified in EPA's OBD Guidance, Appendix D. These vehicles, and others, are also included in EPA's document entitl ed 

"OBD Readiness Testability Issues" (EPA-420-8-1 2-044, June 2012) which updates and replaces the original Appendix D. 

5 
9. Confirmation of Fuel Type: States should evaluate the effectiveness of their current 

procedures for identifying vehicle "fuel type" so that gasoline and diesel vehicles are 

tested based upon the appropriate testing criteria. Methods for confirming vehicle fuel 

type include enhanced Vehicle Identification Number (YIN) decoding and/or completing 

additional OBD requests (PID $51, or PID $01 /Byte B/bit3) while scanning the vehicle. 

LD and MD Diesel IM Readiness Evaluation Recommendation 

Figure 1 provides a test logic flowchart for performing an IM OBD test on diesel vehicles and is 

designed to cover OBD equipped diesel vehicles ::; 14,000 lbs. GVWR. The process logic 

follows the path described below. The recommendations listed in 3a and 3b are those that 

programs should be able to implement immediately as they do not require software or hardware 

upgrades to read Permanent-DTCs from newer vehicles. 

I. Follow all other aspects of inspection protocol prior to readiness evaluation (e.g. visual 

emission-control inspections and inspection of the MIL status)9

• 

2. If the vehicle supports Permanent-DTCs and the state's IM program equipment can 

access the Permanent-DTCs, then the Permanent-DTCs should be used in conjunction 

with readiness monitor status and other criteria such as distance traveled and/or number 

of warm-ups since the DTCs were cleared to make a readiness determination 10• 

3. If the vehicle does not support Permanent-DTCs or the state's IM program equipment 

cannot access the Permanent-DTCs, then the procedures below may be implemented 11

• 

However, in cases where the vehicle supports Permanent-DTCs but the state's equipment 

cannot read them, it should be noted that this method is less robust and should only be 

used as an interim solution until the IM equipment can be updated because these options 

will likely lead to relatively greater motorist inconvenience in some cases and reduced 

emission benefits in other cases. 

a. If the vehicle has ::;1 monitor not ready, then the vehicle passes the readiness 

determination 12 • lfthe vehicle does not meet this criterion, then move on to the 

readiness criteria in Step (b) below to provide relief as appropriate to diesel 

vehicles mapped to the new diesel readiness indicators. 

b. Use the less stringent readiness monitor criteria of 2 not ready monitors in 

conjunction with a more rigorous (as compared to #2 above) distance traveled 

9 As described in the Background section of this document, diesel vehicles may require additional time compared to gasoline 

vehicles to set readiness monitors after a repair. To address this difference, this document recommends that vehicle operators be 

instructed to drive vehicles for two or three weeks after codes are cleared to provide for readiness monitor setting before an 

inspection as opposed to the one week that is often used in gasoline applications. 10 The terms " fail readiness determination" or " fail readiness check" are used here and throughout this document to refer to a 

vehicle that does not meet a given set of readiness criteria. The final 1M test outcome for such vehicles can vary on a state-bystate basis depending on their regulatory requirements and each state should proceed accordingly to fail for readiness, reject from 

testing, etc. 11 This is the path described below in #2 "No use ofPermanent-DTCs and I unset monitors". 12 This is the current readiness criteria used for gasoline vehicles. 

6 
and/or number of warm-ups since the DTCs were cleared if supported 13 • ln order 

to pass readiness in this step, both of the unset readiness monitors must be within 

the categories of PM Filter, NOx/SCR aftertreatment or NMHC Catalyst for 

vehicles mapped to the new diesel readiness indicators (i.e., MY 20 I 0+ diesel 

vehicles), and within the categories of Catalyst, Heated Catalyst, or 02 Heater for 

those that are not. 

The requirement that a minimum distance be traveled or a minimum number of warm-ups occur 

since the last time a vehicle's OBD codes were cleared (both of which can be read directly by 

technicians with a scan tool) 14 

can best be communicated to individual drivers that fail to meet 

these criteria in terms of their needing to operate the vehicle normally for a prescribed minimum 

amount oftime before returning for re-inspection. ln the options outlined below, the workgroup 

has recommended two or three weeks as a good initial starting point for the paths outlined in 

Figure 1, however, each state has the discretion to modify these times following consultation 

with EPA. 

These options and associated paths are outlined in Figure 1 and described in more detail below. 

I. Use of Permanent-DTCs (right-hand side of Figure I) 

It should be noted that implementing the recommendations of this document, including using 

Permanent-DTCs, will likely require revision to current IM inspection software and associated 

databases. For example, the 1M scan tool will need to use a different command to read 

Permanent-DTCs; the electronic inspection record will need to be revised to include PermanentDTC related data; the IM database must be adapted to store the additional data; and some states 

may even change the program's vehicle inspection report (VIR) to identify the presence of a 

Permanent-DTC as the cause ofthe IM failure. Full implementation ofthe recommendations 

will likely vary from state to state as individual states will need to take into account their current 

contract structure and timing, equipment capability, and even associated regulatory changes 

necessary to identify the criteria in regulation. However, as noted above, interim 

implementations that do not use Permanent-DTCs will be less effective and likely have increased 

motorist inconvenience for some vehicles. 

The Permanent-DTC option is illustrated on the right side of Figure I with the key decision 

blocks colored in blue. If a vehicle is presented for testing and the MIL is not commanded on, 

the scan tool will query the OBD system to determine if the vehicle supports Permanent-DTC 

technology. If it does, and there are Permanent-DTCs stored, then the vehicle should be checked 

to see if it has any known testability issues. If it does not, then the vehicle will fail the readiness 

check (unless the vehicle has a known testability issue that would make the failure determination 

inappropriate at this point). 

13 lfthe scan tool or vehicle software does not support checking miles traveled and number of warm-up cycles since code 

clearing, then the vehicle may also pass the readiness determination at the state's discretion. 14 This information is stored in PIDs $30 and $3 1. Temporary disconnection of the battery may cause these counters to reset to 

zero. Vehicles with multiple controllers may report different values for the number of warm-up cycles and distance traveled. In 

such cases, the minimum values reported should be used to determine if the recommended target values fo r these parameters have 

been met. These recommended targets are discussed in more detail below. 

7 
If the vehicle supports Permanent-DTCs and there are no Permanent-DTCs stored, there is higher 

confidence that the vehicle likely does not have an unrepaired fault. There is no recommended 

limitation on the types of2 or fewer incomplete readiness monitors 15 in the second blue decision 

block to determine whether to check the vehicle for known testability issues or move to the next 

blue decision block checking for I or 0 incomplete readiness monitors. 

While the absence of a stored Permanent-DTC does greatly increase the confidence that the 

vehicle does not have an unrepaired fault present, there are still a few scenarios where faults 

could be present without a Permanent-DTC stored (e.g., a recent retlash or reprogramming ofthe 

vehicle, or multiple sequentially monitored components that have failed but have been disabled 

due to the first fault 16). As such, readiness monitors are still used as a backstop to help address 

this concern in the third blue decision block, where, if the vehicle has one or fewer unset 

readiness monitors, then it should pass the readiness determination. 

If there are two unset readiness monitors, then in the fourth blue decision block it is determined 

if the distance since the last code clear is more than 500 miles17 and if the number of warm-ups 

since code clear (WU-SCC) is greater than thirty 18 • If both of these criteria are met, then the 

vehicle should pass the readiness determination. If not, the process should proceed to the fifth 

blue decision block, where it is determined ifthe vehicle has known testability issues 

This will require programs to track and tabulate those specific vehicles (i.e. Make/Model/MY, 

YIN strings) that have demonstrated repeated known readiness problems. The workgroup 

recommends such a list be compiled and maintained by a group of states performing OBD diesel 

testing in consultation with EPA. The list could be distributed through posting on the OBD 

Clearinghouse web site and at the IM Solutions Conference or other public forums. 

2. No use ofPermanent-DTCs and < I unset monitors Cleft-hand side of Figure 1) 

If a diesel vehicle does not support Permanent-DTC technology (or if a program's 1M scan tools 

cannot request Mode $0A data), then a readiness criteria of S l unset monitor is recommended. 

This is the first green decision block to the left of Figure 1. Although many MY 2009 and newer 

diesel vehicles may have difficulty meeting the current gasoline readiness criteria of one or fewer 

unset readiness monitors if codes were recently cleared, some may be able to pass this criterion 

and if they can it is recommended that they pass the readiness check. 

15 The less stringent criterion of any 2 was chosen because the diesel Perrnanent-DTC path in the test logic flowchart is a more 

stringent test than the other option described below. The other option requires more stringent distance and warm-up criteria. 16 With most OBD systems, relevant monitors may become disabled upon detection of a fault with one or more related 

components. If the vehicle owner ignored the detected fault for a substantial period of time, other components could have 

subsequently malfunctioned but will not be monitored until the first malfunction has been repaired. Therefore, requiring some or 

all readiness monitors to be complete wi ll increase the likelihood that the vehicle is not in a condition to trigger a "chain" of 

successive faults after the first one is fi nally repaired. 17 This figure is based on an estimated annual driving distance of 12,000, or I ,000 miles per month. Therefore, it is anticipated a 

vehicle will be driven on average about 500 miles in two weeks and 750 miles in three weeks. The vehicle owner should be 

instructed that normal operation on both city and highway routes is most effective in setting readiness monitors. 18 The value of30 warm-up cycles is based on an assumed average of2 warm-up cycles per day or 14 per week. The workgroup 

recommends using 30 warm-up cycles to represent an average fo r a two week period and 45 warm-up cycles for a three week 

period. 

8 
If the number of unset monitors is greater than one, the vehicle moves to the first yellow decision 

block where if there are three or more unset monitors of any type the vehicle should fail the 

readiness check. However, if there are two unset monitors and both are for monitored controls 

that are known to be difficult to set (i.e., the PM Filter, NOx/SCR after-treatment, or NMHC 

catalyst), then in the second yellow decision block it is determined if more than 750 miles have 

been driven and at least 45 warm-up cycles have occurred since the last code clear 19

• If these 

conditions are met then the vehicle should pass the readiness determination. If the scan tool or 

vehicle software does not support checking miles traveled and number of warm-up cycles since 

code clearing, then the vehicle may also pass the readiness determination at the state's discretion. 

Otherwise, the vehicle should fail the readiness check. 

The criteria in this scenario are more stringent than in the case where Permanent-DTCs are 

supported primarily because of the higher confidence the absence ofPermanent-DTCs provides 

in assessing whether the vehicle has an emissions related problem. Furthermore, older vehicles 

that do not support Permanent-DTCs typically have fewer readiness monitors. Therefore, it is 

appropriate to further limit which monitors can be incomplete and to require additional vehicle 

operation time since the last code clear to help ensure that fault codes have not been erased 

without the owner having performed the necessary repairs. 

19 The value of2 unset monitors among this group was chosen based on an analysis of existing data from Massachusetts that 

demonstrated using 2 unset monitors (regardless of which ones they were) instead of I resulted in LDD vehicle Make/Model 

categories with initial not ready rates in the range of0-9%. Using the :S I criteria resulted in initial not ready rates in the range of 

0-13%. The overall MD diesel vehicle initial not ready rate fell from 7% to 3% with the ::::2 criteria. This analysis is based on 

5,817 LD diesel and 3,232 MD diesel vehicle initial OBD tests performed in Massachusetts on MY 2010-2012 collected in 2010-

2012. It is recommended that similar analyses be performed as additional OBD diesel data becomes available to confirm the 

validity of these criteria or provide information that will support adjusting the criteria accordingly. 

9 
1· Tht.e p.ths may tl10 be 1.1sed kl I:M Interim for 

c1seswhere OM IMMction sottw.e does not rnd the 

requiredP¥~rnt\trsuch as 'llf"mmi\Cnt·DTCs. or 

dbtlnc.e lttd ..,.,.m-ups Jfnc.e codt dew. 

2·PM filter, N0s1$CR .,_rtreMmtnt CH NMHC 

catllyst. or wft:hln CU~lpe,. Heated c.tlfY'It. or Head 

O'lmanltDn: if~ vehicle or lnlpec:tlon~ ~ 

~ MY2010 retdNss mr;WlOt deftnltiom. 

3·5t.le1. shouldtrt:lt .,.., Re..,_IS Chec:k'1ike the-y 

curnnttv hlndla psoline whk: ._.thai fall reldiMH 

ch«kfron'lan tdminiStr.avt stwtdpoint: howner~ the 

owner shoukl be 10id tD dr1w for two cw thr••••lcs on riOtfnll dty aftd ~ tOlMS to Mt the monf\ors 

beforeretur'*'l:• oppotad to the one week fOf' 

psoane vehicles. 

4- Somt ••ln!IPIH of pot.ntllf technlcll iuuet ~It 

coold le-.cl to • whlde h:wk\1 dlffkulty meetiftt the 

re.stneucriterilln the futuretnd ,..,_them to be 

tncludedin 1 known Teltabilfty tssues Rn lfe prO'ridcd 

lnAppendbi8. 

All~ctsofin.spt"ctiontothi1 

point follow P5olinc whlcle 

protocots (e ... >Aw1l ln~ctlon, 

flllure to communlnte, YISUIII MIL 

check,. MIL c~ n.tw., fr.ud 

detection, etc:.) Now the whiclto is 

rudy~ bt evllulttd for the 

rucl lnes.se~mtntofthe Inspection. 

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Appendix A: Workgroup Members 

The workgroup members are listed below. 

State: 

California Air Resources Board- Mike McCarthy, Allen Lyons 

Massachusetts Department of the Environment- Craig Woleader, Julie Ross, Paul Davis 

New Jersey Department of Environmental Protection- Bill Etherington, Tom Dvorak 

New York Department of Environmental Conservation- Jim Clyne, Greg English 

Utah Department of Environmental Protection- Joe Thomas, Chris Woodhave 

Industry: 

Cummins- Mark Stepper, Robert Weiss, Greg Pataky 

Ford- Ken McAlinden 

General Motors- Dave Ferris, Chris Brown 

Volkswagen- Bob Gruszczynski, Volker Lantzsch 

BMW- Carl Heinz Bayer, Edwin Bachhelm 

Mercedes- Ashim Manchanda 

Associations: 

Auto Alliance- Steve Douglas 

Engine Manufacturers Association- Lisa Stegink 

Environmental Protection Agency: 

Office of Transportation and Air Quality- Patty Klavon, Dave Sosnowski 

Contractor: 

Eastern Research Group- Mike Sabisch, Diane Preusse, Jim Lindner 

A-I 

l 
Appendix B: Known Testability Issues 

The EPA guidance document "Performing On board Diagnostic System Checks as Part of a 

Vehicle IM Program" released in June 2001 contained an appendix (Appendix D) that identified 

those gasoline vehicles by Make/Model/MY that were known to have difficulty setting readiness 

or may never set certain readiness monitors. The purpose of Appendix D was to alert IM 

programs that these vehicles should be handled differently with regard to the readiness criteria 

when presented for an OBD test. Flexibility was given to states to perform an idle or two-speed 

idle test on these vehicles or perform an OBD test with less than the required number of set 

readiness monitors. Since 200 I, EPA has updated Appendix D to include additional vehicles 

that have been identified with readiness issues. In January 2008 the "IM OBD Vehicle 

Readiness Exception List" was released, and then in June of20 12 the "OBD Readiness 

Testability Issues" document was published. 

Although at this time there is not such a list for diesel vehicles ::; 14,000 lbs. GVWR, some 

vehicles may be identified with OBD readiness testability issues after the procedure described in 

this document and outlined in Figure I have been implemented. There is one "Known Testability 

Issues?" decision block in Figure I as a part of the readiness determination protocol (one path of 

which includes the use of Permanent-DTCs). Two example issues are listed below; however, 

these are only provided for informational purposes so that states can be made aware of the type 

of issues that could arise that are unique to diesel vehicles. As stated earlier in the body ofthis 

document, the workgroup recommends that a Known Testability Issues document be maintained 

by the states and shared through the OBD Clearinghouse and IM Solutions. 

• Example I: If the readiness check failed due to the presence of stored PermanentDTC(s) (right-hand side ofFigure I path): 

If Permanent-DTCs are being utilized as part of the readiness determination protocol, 

the process will fail any vehicle that has one or more permanent DTCs present. This 

is true even if the vehicle has met the readiness criteria. Permanent-DTCs that have 

dependencies on PM trap regeneration or that have environmental dependencies (such 

as ambient temperature) may take a long time to clear after a repair has been made 

and DTCs cleared. Some examples may include PM filter diagnostics, NHMC 

catalyst diagnostics, PM and NOx sensor diagnostics, NOx catalyst diagnostics, DEF 

heater diagnostics, and HC doser diagnostics. If this condition occurs, it may be 

challenging for the driver to get such Permanent-DTCs to clear, possibly taking 

significantly longer than three weeks of driving in order for the vehicle to accrue 

sufficient mileage or see the proper environmental conditions (e.g, the proper 

seasonal conditions) for the monitors to run. 

Vehicles with these types of issues or other types of issues associated with failing 

readiness due to Permanent-DTCs could be included in a Known Testability Issue 

document. 

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• Example 2: If the readiness check failed due to the number of incomplete monitors, 

having two incomplete monitors of the incorrect type, or insufficient distance or 

number of warm-ups since code clear (left hand side of Figure 1 path): 

Some vehicles may have more than two readiness monitors that are difficult to set 

because of dependencies on PM trap regeneration or on operational conditions (such 

as ambient temperature). For example, certain vehicles may have difficulty setting all 

three monitors listed in Footnote 2 of Figure 1, or perhaps have difficulty setting 

monitors not listed in Footnote 2. 

Vehicles with these types of issues or other types of issues associated with failing 

readiness due to the number of incomplete monitors, having two incomplete monitors 

of the incorrect type, or insufficient distance or number of warm-ups since code clear 

could be included in a Known Testability Issue document. 

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