United States Air and Radiation EPA420-R-01-002 

Environmental Protection January 2001 

Agency 

Final Technical Support 

Document for 

“Amendments to Vehicle 

Inspection Maintenance 

Program Requirements 

Incorporating the 

Onboard Diagnostic 

Check”

 Printed on Recycled Paper 
EPA420-R-01-002 

January 2001 

Final Technical Support Document for 

“Amendments to Vehicle Inspection Maintenance 

Program Requirements Incorporating the 

Onboard Diagnostic Check” 

Edward Gardetto, Ted Trimble, Martin Reineman
 

and
 

David Sosnowski (Editor)
 

Transportation and Regional Programs Division 

Office of Transportation and Air Quality
 

U.S. Environmental Protection Agency
 

NOTICE
 

This technical report does not necessarily represent final EPA decisions or positions.
 

It is intended to present technical analysis of issues using data that are currently available.
 

The purpose in the release of such reports is to facilitate the exchange of
 

technical information and to inform the public of technical developments which 

may form the basis for a final EPA decision, position, or regulatory action.
 
TABLE OF CONTENTS 

1.0 Overview
 

2.0 OBD-I/M Pilot 1: OBD Checks and Tailpipe Testing
 

2.1 Summary of Goals and Conclusions
 

2.2 Background
 

2.3 Vehicle Sampling
 

2.3.1 Methodology
 

2.3.2 Results
 

2.4 Vehicle Testing
 

2.4.1 Methodology
 

2.4.2 Results
 

2.4.2.1 Emission Reductions
 

2.4.2.2 OBD and Preventative Maintenance
 

2.4.2.3 OBD and Errors of Omission (“False Passes”)
 

2.4.2.4 OBD and Errors of Commission (“False Failures”)
 

2.5 Conclusions
 

3.0 OBD-I/M Pilot 2: OBD Checks and Evaporative Emission Testing
 

3.1 Summary of Goals and Conclusions
 

3.2 Background
 

3.3 Vehicle Sampling
 

3.3.1 Methodology
 

3.3.2 Results
 

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3.4 Vehicle Testing
 

3.4.1 Methodology
 

3.4.2 Results
 

3.5 Conclusions
 

4.0 OBD-I/M Pilot 3: Analyzing the Wisconsin OBD-I/M Program Experience
 

4.1 Summary of Goals and Conclusions
 

4.2 DLC Location
 

4.2.1 Background
 

4.2.2 Results
 

4.2.3 Conclusions
 

4.3 Vehicle Readiness
 

4.3.1 Background
 

4.3.2 Results
 

4.3.2.1 Readiness
 

4.3.2.2 MIL-on and IM240 Failure Rates
 

4.3.3 Conclusions
 

4.4 Gas Cap Testing vs. OBD-I/M
 

4.4.1 Background
 

4.4.2 Results
 

4.4.3 Conclusions
 

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1.0 Overview 

On September 20, 2000, EPA proposed to revise existing Motor Vehicle 

Inspection/Maintenance (I/M) requirements related to the incorporation of Onboard Diagnostic 

(OBD) checks into such programs. Among other things, the proposed regulatory revisions --

once adopted -- will accomplish the following: 

1) Allow the OBD-I/M check to replace tailpipe and evaporative system 

testing on OBD-equipped vehicles (with the exception of the gas cap evaporative 

system test); and 

1) Revise the failure and rejection criteria for the OBD-I/M check. 

This Technical Support Document (TSD) provides EPA’s technical justification for these 

amendments, based upon the Agency’s findings gathered during three separate OBD-I/M pilot 

studies. These three pilot studies focused on the following aspects of OBD-I/M testing: 1) OBDI/M’s effectiveness as compared to existing exhaust emission testing; 2) OBD-I/M’s 

effectiveness in identifying faults in the evaporative system; and 3) the unique implementation 

issues associated with incorporating checks of the OBD system into a traditional I/M setting. 

The results of EPA’s pilot testing were shared while still in progress with members of the OBD 

workgroup of the Mobile Source Technical Review Subcommittee established under the Federal 

Advisory Committee Act (FACA). The OBD workgroup’s membership includes representatives 

from the testing and repair industries, vehicle manufacturers, the states, EPA, scan tool 

manufacturers, the academic community, private consultants, and providers of OBD technician 

training. Feedback from the workgroup was used to help guide the progress of the pilots, to 

interpret the results along the way, and to develop the proposed amendments to the rule’s 

existing OBD-I/M requirements. 

This TSD is divided into four main sections. Following the overview section are three 

main sections that coincide with the three pilots identified above. 

2.0 OBD-I/M Pilot 1: OBD Checks and Tailpipe Testing 

2.1 Summary of Goals and Conclusions 

Between September 1997 and October 1999, EPA recruited 201 in-use MY 1996 and 

newer OBD-equipped vehicles and performed an IM240 transient test, an OBD-I/M inspection, 

and an abbreviated version of the vehicle certification test known as the Federal Test Procedure 

(FTP) on each vehicle1 . Vehicles identified as needing repairs were repaired after this initial test 

1 Because the focus of this pilot was comparing OBD-I/M checks to more traditional, tailpipe-based I/M tests, only the 

tailpipe portion of the FTP was performed for this pilot study. The evaporative emission portion of the FTP was performed on a 

smaller sample of vehicles included in the separate, OBD evaporative pilot discussed in section 3 of this TSD. 

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sequence and then subjected to the same sequence again, after repairs. The goal of this test 

program was to answer the following question: 

Is it necessary to conduct both an OBD-I/M check and the IM240 (or some other 

tailpipe test) on OBD-equipped vehicles? 

To answer this question, EPA had to determine if the OBD-I/M check: 1) failed vehicles it 

should have passed; 2) passed vehicles it should have failed; and 3) whether the rate at which it 

did either of these was higher than, lower than, or the same as the IM240. Since it is widely 

considered the most accurate tailpipe-based I/M test available, the IM240 was used to represent 

the “best case” scenario with regard to tailpipe testing in general. 

Based upon the test results detailed in this TSD, EPA concluded that it should not require 

both an OBD-I/M check and the IM240 (or other tailpipe test) on MY 1996 and newer OBDequipped vehicles2 . Specifically, EPA found that while the OBD-I/M check did falsely pass and 

falsely fail some vehicles, in both cases the percentage of vehicles impacted was smaller than 

would be the case with the IM240 and other tailpipe tests. Furthermore, even though the IM240 

caught some of the very few vehicles missed by the OBD-I/M check, the additional cost that 

would result from subjecting OBD-equipped vehicles to two tests instead of one will likely 

outweigh any additional benefit that may be achieved. Lastly, the emission reductions available 

from basing repairs on the OBD-I/M check appear to be at least as large as the emission 

reductions obtained from IM240-triggered repairs on OBD-equipped vehicles. 

2.2 Background 

Under the Clean Air Act as amended in 1990 (CAA), EPA was required to promulgate 

two categories of regulations related to OBD. The first regulated vehicle manufacturers and 

required the installation of the OBD system on all new light-duty vehicles and light-duty trucks. 

The second regulated state I/M programs and required that all such programs – whether basic or 

enhanced – include an inspection of the OBD computer for vehicles so equipped. In 1992, when 

EPA published its original I/M rule, federal OBD certification requirements were still being 

developed, and so sections were reserved in the I/M rule to address the OBD-I/M testing 

requirement at a later date. Since the 1992 I/M rule was published, EPA has amended it twice to 

address OBD-I/M testing requirements – first, on August 6, 1996, and again on May 4, 1998. 

In the 1996 amendments, EPA described how OBD was to be addressed as part of the 

basic and enhanced I/M performance standards and established OBD-I/M SIP requirements. The 

1996 amendments also specified data collection, analysis, and summary reporting requirements 

for the OBD-I/M testing element; established OBD test equipment requirements and the OBD 

test result reporting format; and identified those conditions that would result in either an OBD2 Although EPA does not intend to require dual testing of OBD-equipped vehicles for reasons detailed in this TSD, 

states wishing to dual test MY 1996 and newer vehicles may do so. EPA will work with individual states to determine whether 

or not additional credit is warranted on a case-by-case basis. 

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I/M failure or rejection. Lastly, the 1996 amendments established January 1, 1998 as the 

deadline by which most I/M programs were to begin OBD-I/M checks, though vehicles were not 

required to be failed based upon the OBD-I/M check until January 1, 20003 . The data gathered 

by state programs between January 1, 1998 through December 31, 1999 was to be used to assess 

the effectiveness of the OBD-I/M check relative to the IM240. 

Subsequent to the 1996 amendments, the I/M test environment changed significantly, 

with the result that use of the IM240 was not as prevalent as had once been expected. In the 

same time frame, EPA discovered that the IM240 test as originally designed might lead to false 

failures for some vehicles due to insufficient preconditioning4, and as a result the test itself might 

not be as effective as once thought. This latter issue suggested that evaluating the OBD-I/M 

check based upon a comparison to the IM240 could unfairly penalize the OBD-I/M check. 

Members of the OBD workgroup5 (established under the Federal Advisory Committee Act) 

raised similar concerns regarding the appropriateness of comparing the OBD-I/M check to a “hot 

start” test like the IM240 as opposed to the FTP, which is a “cold start” test and the standard to 

which new cars are certified. Furthermore, OBD design requirements are based in part on 

detecting emission failures which are directly related to the FTP6 

. 

As a result of these changing conditions and concerns, EPA revisited its original plans for 

evaluating the effectiveness of OBD-I/M testing by comparing it to state-gathered IM240 

inspection lane data. Instead, EPA decided to pursue the test program described here, in order to 

alleviate the need for states to run dual tests (tailpipe and OBD) in their I/M lanes merely as a 

form of data gathering7 . The May 4, 1998 amendments to the I/M rule addressed this change by 

delaying the date by which I/M programs were to begin OBD testing to no later than January 1, 

20018 . To generate the necessary data for comparison, EPA and its research partners conducted 

sample testing at four different labs: the National Vehicles and Fuels Emissions Laboratory 

(NVFEL) in Ann Arbor, Michigan; the Automotive Testing Laboratory (ATL) in Mesa, Arizona; 

the Colorado Department of Health Laboratory (CDH) in Aurora, Colorado; and the California 

Air Resources Board (CARB) test facilities in El Monte, California. 

3 Programs qualifying for the Ozone Transport Region (OTR) low enhanced performance standard were allowed to 

postpone mandatory OBD-I/M testing until January 1, 1999. 

4 SAE paper 962091, “Preconditioning Effects on I/M Test Results Using IM240 and ASM Procedures,” Heirigs, 

Philip; Gordon, Jay. 

5 The OBD workgroup is a subgroup of the Mobile Sources Review Subcommittee, which was itself formed under the 

Federal Advisory Committee Act (FACA) in order to advise the Agency on technical matters. 

6 The exhaust standards for OBD require that a dashboard light be illuminated under circumstances which could lead 

the vehicle to exceed its certification standards by 1.5 times the standard. 

7 Federal Register Volume 61, No. 152; August 6, 1996; page 40940. 

8 In its September 20, 2000 notice of proposed rulemaking, EPA proposed to extend this deadline to January 1, 2002 in 

addition to the other revisions discussed in this TSD -- in part due to the proximity of the current deadline to the release of these 

findings and the proposed amendments. 

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2.3 Vehicle Sampling 

2.3.1 Methodology

 Based upon advice from the Mobile Source Review Subcommittee, EPA decided to 

conduct an FTP-based test program with a minimum of 200 vehicles9 . The goal of the test 

program was to determine how well OBD-I/M testing compared to tailpipe I/M testing. Because 

the IM240 is generally accepted as the most accurate tailpipe test available for use by I/M 

programs, EPA decided that a comparison to the IM240 would be considered a “best case” 

comparison for establishing relative tailpipe test effectiveness1 0 . 

The recruitment of vehicles for pilot testing was controlled by the need to answer two 

basic questions concerning the effectiveness of OBD-I/M testing relative to traditional tailpipe 

tests: 1) Do vehicles identified by the OBD-I/M check actually need repair, and 2) Does the 

OBD-I/M check miss high emitters that would be caught by traditional tailpipe testing? To 

address the first question, EPA recruited vehicles identified by OBD as possible high emitters in 

need of repair (i.e., vehicles with the malfunction indicator light -- or MIL -- illuminated). To 

address the second question, EPA focused on those vehicles that failed a properly preconditioned 

IM240, but for which no MIL was illuminated. 

Concern about the relatively small sample size and the degree to which it would represent 

the fleet at large led EPA to weight its sample based upon manufacturer production for the six 

largest producers. The remaining manufacturers represent a small percentage (<10%) of the 

entire fleet and are represented by the category “other.” The sample was also weighted to 

account for the growing fraction of light-duty trucks (LDTs) in the fleet. Table 1 below was 

developed to act as a target for the 200 vehicle sample based on 1997 vehicle sales1 1 . 

Table 1: Procurement Goals Based on Production 

MFR GM Ford 

DaimlerChrysler Toyota Honda Nissan Other Total 

LDV 35 21 10 11 11 7 10 105 

LDT 27 29 20 5 1 3 10 95 

Total 62 50 30 16 12 10 20 200 

9 Mobile Source Review Subcommittee meeting of 7/16/97. 

10 Sierra Research Report under EPA Contract 68-C4-0056; WA 2-03; “Development of a Proposed Procedure for 

Determining the Equivalency of Alternative Inspection and Maintenance Programs,” page 7. 

11 Automotive Industries; February, 1998, page 17. 

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For vehicles with the MIL illuminated, any vehicle with a non-evaporative, emissionsrelated trouble code commanding the MIL on was accepted into the program1 2 . These vehicles 

were selected without knowledge of their tailpipe emissions. However, because misfire codes 

are relatively common among the MILs observed in the field, an upper limit of 25% was 

established for misfire codes per manufacturer represented in the overall sample. This 25% limit 

was derived from a fleet survey of over 160,000 vehicles in Wisconsin and represents the relative 

occurrence of misfire diagnostic trouble codes (DTCs) seen in the I/M lane1 3 . 

The pilot study also called for the recruitment of vehicles with (potentially) high 

emissions and no MIL illumination. These no-MIL/high-emitter vehicles were identified and 

recruited based upon two primary criteria: 1) High LANE2401 4 test results or 2) other 

characteristics which experience suggested would result in high emissions (i.e., high mileage, 

and/or driveability problems). Using the first criteria, the most stringent IM240 standards1 5 were 

applied even though the actual state I/M program did not fail vehicles based on those values. 

Additional, potential high emitters were recruited based upon very high mileage, or a 

mechanic’s report that a particular vehicle was running poorly. Because NVFEL is not located 

near an operating I/M program, the Ann Arbor lab used this secondary method as its primary 

means for identifying and recruiting no-MIL/high-emitter vehicles for subsequent testing. ATL 

and CDH also attempted to find additional no-MIL/high-emitter vehicles based upon these more 

qualitative (as opposed to quantitative) criteria. On the vehicles with suspected high emissions 

but no lane-based tailpipe data, the LAB240 was again used to screen which vehicles were kept 

in the sample and which were released from further participation in the pilot study. 

2.3.2 Results 

201 vehicle test slots were filled during the program versus the 200-vehicle target (2 

vehicle slots were filled by the same vehicle, which was recruited twice, six months apart, with 

different problems each time). Table 2 represents the breakdown of this sample by 

manufacturer and is also segregated into cars (LDVs) versus trucks (LDTs). The category of 

“other” is made up of the following manufacturers followed by the number of sample vehicles 

12 “Recommended Practice for Diagnostic Trouble Code Definitions,” SAE J2012, Society of Automotive Engineers, 

Inc., revised March 1999. 

13 “Analyses of the OBDII Data Collected from the Wisconsin I/M Lanes,” Trimble, Ted, Environmental Engineer, 

U.S. EPA, August 2000. 

14 EPA distinguishes between “LANE240" tests (i.e., those conducted by a commercial testing contractor as part of the 

routine operation of an existing program) and “LAB240" tests (i.e., those conducted under controlled, laboratory conditions for 

test type comparison and evaluation purposes). More information concerning the differences between “LANE” and “LAB” 

IM240s is available in Appendix 3. 

15 “EPA I/M Briefing Book: Everything You Ever Wanted to Know About Inspection and Maintenance,” EPA-AAESPD-IM-94-1226, Section 4, page 10. U.S. EPA, Office of Air and Radiation, February 1995. 

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from each: Mazda (2), Volkswagen (3), Isuzu (2), Hyundai (3), Kia (1), Saab (1), Volvo (1), and 

Suzuki (3). Of the 201 vehicles in the sample, 193 were recruited as MIL-on vehicles, while the 

remaining 8 were recruited as no-MIL/high-emitter vehicles. 

Table 2: Description of Sample by Manufacturer and Type 

MFR GM Ford 

DaimlerChrysler Toyota Honda Nissan Other Total 

LDV *45 

**(128%) 

31 (148%) 22 

(220%) 

5 

(45%) 

8 

(73%) 

7 

(100%) 

14 (140%) 132 

(126%) 

LDT 18 

(66%) 

28 

(96%) 

16 

(80%) 

1 

(20%) 

0 

(0%) 

4 

(133%) 

2 

(20%) 

69 

(73%) 

Total 63 

(102%) 

59 (116%) 38 

(127%) 

6 

(38%) 

8 

(67%) 

11 (110%) 16 

(80%) 

201 

(100%) 

* = number procured ** = percent of goal 

The sample breakout by model year and by minimum, maximum, and average odometer 

readings are listed in tables 2a and 2b, respectively. 

Table 2a: Breakout by model year 

1996 1997 1998 1999 2000 

LDV 28 33 38 32 1 

LDT 27 22 14 6 0 

Table 2b: Odometer readings 

LDV LDT 

MINIMUM 29 3,981 

AVERAGE 26,440 54,505 

MAXIMUM 93,575 245,000 

As the results of these procurement efforts are considered, it is important to keep in mind 

the relatively low age of the fleet of vehicles being evaluated (i.e., MY 1996 and newer). The 

relatively low age (four years old or newer) and mileage (average = 37,000 miles) of the vehicle 

population targeted by the study led to fewer MIL illuminations in the general fleet than would 

be expected. We do not believe, however, that the relative newness of the vehicles that 

participated in this pilot will change the direction of the conclusions drawn from this study. 

Specifically, we do not believe that OBD systems will prove somehow less effective at 

identifying vehicles in need of repair as the OBD-equipped population ages. This is because the 

OBD system itself (as opposed to the hardware the OBD system monitors) is primarily a selfcontained, software-based system and not likely to be subject to substantial degradation due to 

aging. The practical impact of the newness of these vehicles was the limited exposure of the 

hardware being monitored to the real world effects of heat, cold, water, salt, etc. However, 

because the possibility exists that multiple-component aging may have a negative, synergistic 

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effect on OBD’s ability to detect vehicles which have high emissions in the future, continued 

study of the OBD-equipped fleet as it ages and accumulates mileage seems warranted1 6 . 

2.4 Vehicle Testing 

2.4.1 Methodology 

During the two-year period from September 1997 through October 1999, EPA and its 

research partners conducted sample testing at four different laboratories across the country: the 

National Vehicles and Fuels Emissions Laboratory (NVFEL) in Ann Arbor, Michigan; the 

Automotive Testing Laboratory (ATL) in Mesa, Arizona; the Colorado Department of Health 

Laboratory (CDH) in Aurora, Colorado; and the California Air Resources Board (CARB) test 

facilities in El Monte, California. FTP testing was performed using the methods described in 

CFR 86.130-96 with the exception that no diurnal heat build or SHED testing was conducted1 7 . 

IM240 testing was done in accordance with EPA Technical Guidance EPA-AA-RSPD-IM-98-1. 

OBD information was gathered using scan tools from various manufacturers complying with the 

standards established by SAE 1978. Maintenance was performed at either the original 

manufacturer’s dealership or by mechanics following the manufacturer’s available service 

information. 

MIL-on vehicles were inspected when they first arrived at the lab using the LAB240 

procedure and the fuel that was already in the vehicle’s tank. The tanks were then drained of inuse fuel and refilled with indolene test fuel. The vehicles then received a standard FTP and a 

second LAB240. This provided the “As-Received” emissions profile of the vehicle. The FTP 

was the standard for comparing actual emissions reductions; the IM240 and the OBD-I/M test 

results were only used to identify vehicles as either “pass” or “fail,” relative to the respective I/M 

test type. Vehicles identified as “failures” based upon either their tailpipe or OBD-I/M results 

were then sent for repairs, after which they were again tested on the FTP to determine their “after 

repair” emission levels. Any difference measured between the two FTPs represented the air 

quality improvement1 8 or emission benefit. Most maintenance was performed following original 

equipment manufacturer (OEM) published procedures, while in some cases, this information was 

supplemented through consultation with OEM engineers. In cases where DTCs were present but 

16 In recognition of the potential impact of high mileage on OBD effectiveness, EPA recently completed testing and 

has begun analyzing the results from a study of 43 OBD-equipped vehicles with mileages of approximately 100,000 miles to as 

high as 273,000 miles. Early indications suggest that high mileage does not have a noticeable impact on the effectiveness of the 

OBD system to detect needed repairs. With regard to the impact of simple aging, EPA recognizes the value of gathering 

additional information on the durability of OBD systems as they age, and stands ready to revise the OBD-I/M requirements 

should future study suggest such is warranted. 

17 Sealed Housing for Evaporative Determination (SHED) testing was conducted on a different subset of vehicles, as 

part of the evaporative system pilot, which is addressed in section 3.0 of this TSD. 

18 Two vehicles were too dirty and/or running too poorly to test on the FTP. Since it was not possible to establish an 

accurate “before repair” emission measurement for these vehicles, no air quality benefit was assigned to them. (See discussion in 

Table 10, and Appendix 4). 

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the mechanics could find nothing in need of repair or replacement, the OBD system was allowed 

to reset and was then monitored to verify the absence of any OBD- or emission-related problem. 

(See Appendix 1 for additional test sequence details.) 

2.4.2 Results 

Table 3 shows the number of MIL-on vehicles with initial FTP readings 

exceeding 1 and 1.5 times the applicable certification standard (the latter being a subset of the 

former). One and a half times the standard was chosen as a criterion for comparison because the 

certification requirements for OBD specify that the MIL is to be illuminated if a problem is 

detected that could result in emissions exceeding 1.5 times the exhaust certification standard. 

Table 3 also shows the subset of MIL-on vehicles for which the MIL cleared on its own, after 

being recruited but before any repairs could be attempted1 9 . 

Table 3: MIL-On Vehicles vs. FTP and 1.5 Times FTP 

MIL on MIL self-cleared FTP > 1 times cert. FTP > 1.5 times cert. 

LDV 128 5 40 21 

LDT 66 6 18 10 

Total 194 11 58 31 

As noted in footnote 18, Table 3 includes two vehicles which are assumed to have failed 

their as-received FTP at over 1.5 times the applicable tailpipe standards. These vehicles could 

not be driven on the FTP trace and therefore no tailpipe readings are available. A description of 

these two vehicles is included in Appendix 4. 

EPA also recruited vehicles with suspected high emissions but no MIL illumination. 

Using the screening methods discussed earlier, 8 vehicles qualified to represent this category. 

Table 4 shows the number of MIL-off vehicles with initial FTP readings exceeding 1 and 1.5 

times the applicable certification standard (the latter being a subset of the former). 

Table 4: MIL-Off Vehicles vs. Certification and 1.5 Times Certification Standard 

MIL-off FTP > 1 times cert. FTP > 1.5 times cert. 

LDV 4 2 1 

LDT2 0 4 3 3 

Total 8 5 4 

19 MIL self-clearing is a design feature of OBD systems, and is the way the system accounts for intermittent problems 

(like misfire) that may occur once under atypical vehicle operation, but then seem to disappear during more normal driving. This 

aspect of OBD is discussed in more detail in section 2.4.2.2, “OBD and Preventative Maintenance.” 

20 All three figures in this row include vehicle CDH04, which was recruited with no MIL but is considered an accurate 

OBD identification (see Appendix 4). 

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The ability of OBD to correctly identify vehicles which are emitting at levels 

significantly over their applicable certification standard (2 times or higher) was also investigated. 

The subgroup of vehicles making up this sample is listed in Table 5 below. 

Note that OBD-I/M missed two of a total of 21 vehicles identified as gross emitters with 

FTP scores of two or more times their certification standards (i.e., OBD successfully identified 

90% of these gross emitters). Note further that the LAB240 missed four times as many vehicles 

from this category, and identified only 62% of the grossest emitters. The two vehicles missed by 

OBD-I/M -- one LDV (CDH03) and one LDT (CDH33) -- were correctly failed by the LAB240, 

while the eight LDVs missed by the LAB240 were correctly failed by OBD-I/M. 

Table 5: Vehicles with FTP Results Over 2 Times the Certification Standard 

As measured by FTP Identified by OBD Identified by LAB240 

LDV 15 14 7 

LDT 6 5 6 

EPA also collected data on the degree to which repairing vehicles solely to turn the MIL 

off resulted in emission reductions that changed FTP-failing vehicles into FTP-passing vehicles. 

Of the 15 LDVs with emissions over twice their standard (>2xFTP), 12 (or 80%) were repaired 

to below certification levels by targeting repairs solely at correcting the conditions that led to the 

MIL being on. All 14 of the >2xFTP LDVs identified by OBD-I/M that were repaired based 

upon OBD-targeted repairs tested below 1.5 times the applicable standard after those repairs 

(i.e., below the minimum required detection threshold for OBD). The two vehicles that remained 

above the FTP standard (but below 1.5 times that standard) after repairs to turn off the MIL are 

discussed in section 2.4.2.3 of this TSD (Table 9). 

2.4.2.1 Emission Reductions 

The emissions reductions attributable to OBD- and LAB240-triggered repairs performed 

as part of this pilot are presented in Table 6 below. Vehicles which failed both the LAB240 and 

OBD-I/M tests are included in both the IM240 and OBD categories of repair data presented 

below. Nevertheless, vehicles that failed both the LAB240 and OBD-I/M tests were repaired 

based mainly on the OBD codes and therefore the IM240 repair data are not completely 

independent of OBD effects. Wholly separate from its use in the I/M arena, OBD is a powerful 

tool for diagnosing and repairing vehicles in the real world, and more and more repair 

technicians are using the OBD scan as their starting point for diagnosing vehicles prior to repair. 

Although EPA could have required technicians to ignore OBD when attempting to fix vehicles 

identified by IM240 for repair, we believe that such a restriction would be artificial and 

unnecessarily limiting. 

The varying sample sizes listed above for non-methane hydrocarbon (NMHC), total 

hydrocarbon (THC), and CO2 are due to the fact that while NVFEL and ATL measured all five 

pollutant categories for the pilot vehicles they tested, the CDH did not measure NMHC and 

CARB did not measure THC or CO2 for their respective vehicles. The THC and NMHC 

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averages quoted below are therefore based upon the subset of vehicles for which those emissions 

were measured. LDV and LDT data are presented separately because of significant differences 

in their certification standards and control strategies. 

Table 6: Average Reductions and Fuel Economy Improvement from OBD vs. IM240 Repairs 

LDV 

THC 

(gpm) 

NMHC 

(gpm) 

CO 

(gpm) 

NOx 

(gpm) 

CO2 

(gpm) 

FE Increase 

(mpg) 

Avg. OBD 

n=126 

0.138 

n= 114 

0.1 

n= 111 

2.4 

n= 126 

0.1 

n=126 

6.47 

n=114 

0.53 

n=114 

Avg. IM240 

n=7 

1.04 

n=7 

0.9 

n=5 

15.4 

n=7 

0.6 

n=7 

14.71 

n=7 

2.36 

n=7 

LDT 

Avg. OBD 

n=65 

0.11 

n= 65 

0.05 

n= 49 

1.56 

n=65 

0.13 

n=65 

-2.66 

n=64 

0.03 

n=64 

Avg. IM240 

n=7 

0.84 

n=7 

0.37 

n=5 

10.47 

n=7 

0.60 

n=7 

8.27 

n=7 

0.79 

n=7 

Another way to look at the same repair reductions is to quantify the total grams per mile 

(gpm) reduced over the course of the study as opposed to average gpm reductions. Table 7 

below quantifies the total gpm reductions attributable to repairs triggered by either OBD or 

IM240, per pollutant category and segregated by LDVs and LDTs. If a vehicle failed both the 

LAB240 and the OBD tests, the gpm reductions resulting from repairs were counted under both 

categories. Note that with the exception of fuel economy improvement on LDTs, OBD-triggered 

repairs consistently produced more total reductions and fuel economy improvement than did the 

IM240-triggered repairs. 

Table 7: Summation of Reductions Associated with OBD vs. IM240 Triggered Repairs 

LDV THC 

(gpm) 

NMHC 

(gpm) 

CO 

(gpm) 

NOx 

(gpm) 

CO2 

(gpm) 

FE Increase 

(mpg) 

OBD reductions 15.7 11.1 298 12.1 737 60 

IM240 reductions 10.0 4.9 277 5.4 25 27 

LDT 

OBD reductions 7.5 2.6 101 8.2 43 2 

IM240 reductions 6.4 1.9 90 7.1 61 6 

It should be pointed out that in its comparison of the emission reductions attributable to 

the OBD-I/M check versus the IM240, the OBD tailpipe study was biased in favor of the IM240 

to ensure that the conclusions drawn regarding the OBD-I/M check’s relative effectiveness were 

conservative. Specifically, when a vehicle was identified as a likely IM240 false failure based 

upon a comparison of LANE240 and LAB240 test results, that vehicle was then dismissed from 

further participation in the study. As a result, the gpm emission reductions attributed to IM240 

were not “watered down” down by the false failures noted between the LANE- and LAB240s. 

Conversely, potential OBD-I/M check false failures were included in the sample and were 

13
 
 

 

 

 

 

 

 

 

 

actively recruited. Therefore, the gpm reductions attributed to either test based upon this pilot 

really do represent the “best case” scenario for the IM240 and the “worst case” scenario for the 

OBD-I/M check. 

2.4.2.2 OBD and Preventative Maintenance 

As a matter of design, OBD should be able to identify the need for repairs and/or 

maintenance prior to actual increased emissions. This is because OBD monitors the performance 

of individual emission control components, several of which may need to fail in sequence, or 

over a period of time before the problem shows up at the tailpipe. For example, a periodic 

misfire might not lead to immediate increases in emissions, but eventually can destroy the 

catalyst, at which time tailpipe emissions will increase substantially (as will the likely cost of 

repairs). Traditional tailpipe tests are less capable of identifying this kind of preventative repair, 

because such tests rely exclusively upon measurement of post-catalyst tailpipe emissions. 

Therefore, with traditional tailpipe tests, a relatively inexpensive problem to begin with may 

become critical before it can be detected. 

The tailpipe pilot evaluated this aspect of OBD, and Table 8 lists the results of 

maintenance performed on vehicles with tailpipe emissions below the applicable certification 

standards. Vehicles for which EPA was unable to identify or reproduce the condition that led to 

the original MIL illumination are designated below as MNR (for “Malfunction Not 

Reproduced”). Vehicles for which the MIL went out on its own after procurement but prior to 

attempted repair are designated below as “MIL self-cleared.” See Appendix 2 for a list of the 

parts replaced on these vehicles OBD identified as needing maintenance. 

Table 8: Maintenance aspect of OBD MIL illumination identification 

MIL on/FTP pass Broken part(s) found MNR MIL self-cleared 

LDV 88 63 25 3 

LDT 48 34 14 6 

Total 136 97 39 9 

In considering these results, it is important to understand that a MIL going out on its own 

is considered a part of the normal operation of the system; it is not necessarily an indication that 

the OBD system itself is having a problem. Every mechanic knows that vehicles are complex 

systems and can experience intermittent or transient problems that seem to go away on their own. 

And most motorists have had the experience of having a problem that mysteriously “goes away” 

the second they take it into the shop. Perhaps the vehicle has been put under an unusually high 

load, the fuel quality is below par, or the vehicle is otherwise being operated under atypical 

conditions. The OBD system is designed to account for intermittent problems by setting a code 

when a problem is first detected (for example, a misfire) and then to monitor the vehicle to see if 

the problem recurs after a certain number of key-on/key-off cycles. If the problem does not 

recur, the system is allowed to extinguish the MIL, though a record of the problem is recorded 

and retained by the OBD system for a certain period of time after the MIL is turned off, 

14
 
 

 

depending upon the nature of the code. The last column of Table 8 above – “MIL self-cleared” – 

represents this particular subset of MNR vehicles recruited as part of the tailpipe pilot. Given the 

difficulty EPA had in finding MY 1996+ vehicles with the MIL on for recruitment, we may have 

tended to procure vehicles as soon as the MIL turned on (and before the OBD system had a 

chance to clear itself if the problem detected was an intermittent or transient condition), which 

does not reflect the anticipated experience of an OBD-I/M program. As a result, EPA believes 

its sample may have been biased in the direction of recruiting vehicles with MILs lit for 

intermittent problems. 

2.4.2.3 OBD and Errors of Omission (“False Passes”) 

During the pilot program, 4 vehicles were found with no MILs illuminated or DTCs set, 

but which nevertheless had tailpipe emissions exceeding both their certification standards and the 

1.5 times the certification level at which OBD is supposed to command the MIL to illuminate 

and set relevant DTCs. A fifth vehicle was also found to have high emissions and no visible 

MIL illumination, though when this vehicle was scanned, it was found that the MIL was, indeed, 

commanded on, but had not illuminated due to a short in the system. All 5 of these vehicles are 

listed in Table 9 below along with a brief summary of the cause(s) of their high emissions. 

The first two vehicles (CDH03 and CDH33) had high emissions and no MIL or pending 

DTCs prior to repair, while the next two vehicles (ATL120 and ATL130) arrived with the MIL 

on. In the case of ATL120, the MIL was extinguished by repairs, but the vehicle still produced 

high emissions after these repairs. In the case of ATL130, a diagnostic scan revealed nothing to 

fix and the MIL did not re-light after being cleared by the scanner, even though the vehicle’s 

emissions were still high after the MIL was cleared. And the last vehicle (CDH04) could not be 

driven on the FTP because it stalled in third gear, but was assumed to be a high emitter due to the 

fact that it produced a plume of black smoke when tested on the LAB240. 

CDH03 is considered an OBD error of omission due to its emission levels and the lack of 

MIL illumination and DTCs. The repair of the oxygen sensor returned this vehicle to acceptable 

emissions level. Further investigation of this problem by Daimler-Chrysler engineers found an 

unanticipated failure mode of the rear oxygen sensor. Daimler-Chrysler found that this failure 

mode would be detected by all later OBD systems in their product line. No additional examples 

of this type of oxygen sensor failure mode were located in this test program. 

15
 
 

 

 

 

 

 

 

 

 

 

 

 

 

Table 9: Discussion of Specific Vehicles 

Vehicle FTP Emissions Problem found 

CDH03: 1996 Chrysler Neon; 

86,236 miles; LANE240 failure 

(As received) 

THC/NMHC/CO /NOx 

FTP: 1.73/xx/52.0/0.25 

OBD error of omission; unanticipated oxygen 

sensor failure; problem fixed for later model years. 

CDH33: 1997 Daimler-Chrysler 

1500 Pick-up truck, odometer 

113,543; LAB IM240 failure 

(As received) 

FTP: 0.55/xx/12.8/2.9 

Standard: xx/0.4/5.5/0.97 

THC was < 1.5 times standard (NMHC unknown) 

but CO and NOx > 1.5 times. See catalyst monitor 

discussion below. 

ATL120: 1997 GM Grand Am; 

47,173 miles; MIL extinguished 

after diagnostics. 

(Post diagnostics; no repair) 

FTP: 0.14/0.12/1.6/0.97 

Standard: xx/0.25/3.4/0.4 

No problem found during diagnostics and MIL did 

not reset after clearing, even though NOx was 

above OBD trigger level. (HC and CO remained 

below the OBD trigger level.) 

ATL130: 1996 Isuzu Hombre 

(GM system); 235,000 miles; MIL 

extinguished after diagnostics. 

(Post repair) 

FTP: 0.5/0.39/17.1/0.6 

Standard: xx/0.31/4.2/0.6 

On post-repair FTP with MIL off, CO was still > 

1.5 times standard, while HC fell below the OBD 

trigger level. See catalyst monitor discussion 

below. 

CDH04: 1996 GM S10 Pickup 

Truck; 27,063 miles; LANE240 

failure 

Could not be driven on FTP; 

projected failure (See Appendix 4) 

MIL commanded on, but electrical short prevented 

illumination; would be caught by OBD-I/M scan. 

The issue with CDH33, ATL130, and ATL120 seems to be a matter of timing and the 

way that catalyst efficiency losses are monitored by OBD2 1 . Currently, catalyst monitors only 

target HC to establish catalyst efficiency2 2 based on the fact that the vast majority of possible 

failure modes leading to increased CO and NOx emissions from the catalyst will also eventually 

lead to increased HC emissions -- at which time a DTC will be set and the MIL illuminated. 

While CDH33, ATL130, and ATL120 showed high emissions for NOx and/or CO, the 

malfunction in question simply had not had time to lead to excessive HC emissions. 

Lastly, CDH04 is not considered an OBD error of omission because the computer was 

commanding the MIL on, but the nature of the problem (a short in the electrical system) 

prevented the MIL from illuminating. This type of problem would be identified as a failure in an 

OBD-I/M program and would be required to be repaired. This vehicle helps illustrate why a 

simple pass-fail visual check for MIL illumination is not enough on which to base an I/M 

inspection; a scan of the onboard computer is also needed to help determine if there is a problem 

with vehicle readiness, a malfunctioning MIL, a short in the wiring, et cetera. 

21 EPA did not perform a detailed analysis of the entire emissions systems on these vehicles. Therefore, we cannot say 

for certain that these CO and NOx problems are exclusively due to loss of catalyst efficiency, though it is our engineering 

judgment that catalyst efficiency is a significant, contributory cause of the results observed. 

22 California Air Resources Board Regulation, “Malfunction and Diagnostic System Requirements, 1968.1(b)(1.2.1-

1.2.4). 

16
 
 

 

 

 

 

 

 

 

 

 

 

2.4.2.4 OBD and Errors of Commission (“False Failures”) 

Of the 194 vehicles that were accepted into the program with the MIL on, 43 or 22% 

were sent home without a repair identified because repair technicians were unable to replicate the 

cause of MIL illumination. Ten of these vehicles were sent home because the light went out 

before initial testing was completed; this was attributed to OBD’s ability to self-clear nonrecurring, intermittent fault codes (as discussed above). Because the repair goal of the tailpipe 

pilot was to extinguish the MIL, no repairs were attempted on these self-clearing vehicles and 

they were dismissed without further testing. 

Of the 33 remaining vehicles, 30 had FTP measurements at or below the applicable 

certification standards, 2 (ATL120 and ATL94) had FTP scores below the OBD threshold of 1.5 

times the certification standard, and 1 (ATL120) was a gross emitter which EPA was unable to 

fix based upon OBD diagnostics. Based upon engineering judgment, EPA believes that the 

majority of these vehicles had intermittent problems that for one reason or another were not 

manifesting themselves at the time repairs were being attempted. Almost half (15) had misfire 

codes, while an additional 11 had fuel trim OBD codes for which OEM diagnostics failed to 

identify a specific cause. Misfires, it should be noted, are notoriously intermittent. In some of 

these cases the repair technicians were able to reproduce the misfire by spraying the engine 

compartment with water. In at least one case, however, the repair technicians were unsuccessful 

with this technique even though they could plainly see where the misfire was occurring from a 

plug wire that was not routed correctly. In one case where the repair technicians were unable to 

reproduce the misfire on their own, the owner took technicians out on the road to demonstrate 

when the misfire occurred (e.g., at high rpm and load, off the FTP cycle). This was a case where 

EPA was able to convert a vehicle that seemed like a potential false failure into one where the 

OBD system successfully identified a vehicle in need of repair. It is possible that other pilot 

vehicles identified as potential OBD false failures based upon the repair technicians’ inability to 

identify a fixable problem could have been successfully repaired if EPA had access to the vehicle 

owners. Such access was the exception rather than the rule during the tailpipe pilot study, 

because in many cases the “owners” of the vehicles were actually car rental agencies or 

dealerships without practical knowledge of the individual vehicles in their fleets. In a real world 

I/M program, the repair technician would be able to consult with the vehicle owner concerning 

the conditions under which the MIL was illuminated, and as a result, the incident of unfixable 

OBD failures should be lower than suggested by the pilot sample. 

2.5 Conclusions 

Based upon the results of the OBD tailpipe pilot, EPA concluded that OBD scanning and 

repair is a viable basis for I/M testing for MY 1996 and newer, OBD-equipped vehicles. The 

emission reductions attributable to OBD-triggered repairs appear to be at least as large as those 

attributable to repairs triggered by the most accurate, traditional I/M tailpipe test (i.e., the 

IM240). In direct comparison to the IM240, OBD-I/M checks identified more vehicles with 

tailpipe emissions exceeding their certification standards. With few exceptions, OBD-I/M 

checks identified the same true failures as did the IM240, while also providing diagnostic 

17
 
 

 

 

 

 

 

 

 

 

 

information for repairing those vehicles. Furthermore, OBD-I/M checks: 1) identified vehicles 

that were falsely failed on the IM240 as clean; 2) identified high emitting vehicles missed by the 

IM240; and 3) identified vehicles with broken or worn components that needed replacement or 

repair prior to the actual development of emissions problems (thereby providing additional air 

quality benefits in the form of pollution prevention). Additionally, EPA found that OBDtriggered repairs effectively returned vehicles to their proper operating conditions and that 

tailpipe emissions returned to below certification levels in the majority of cases. 

3.0 OBD-I/M Pilot 2: OBD-I/M Checks and Evaporative Emission Testing 

3.1 Summary of Goals and Conclusions 

From April 1999 through May 2000, Automotive Testing Laboratories, Inc. (ATL) in 

Mesa, Arizona conducted pre- and post-repair evaporative system emission testing on 30 OBDequipped vehicles under contract to EPA. Unlike the tailpipe pilot described in section 2, the 

evaporative pilot did not use vehicles with naturally occurring system failures. Instead, specific 

purge system malfunctions and evaporative system leaks were induced to see whether or not the 

vehicles’ OBD systems were capable of detecting a range of evaporative system problems. After 

the failures were induced, the vehicles were then tested using the evaporative portion of the FTP. 

Once OBD’s ability to detect these induced evaporative system failures was established along 

with the vehicles’ pre-repair FTP scores, ATL technicians then repaired the vehicles, and a 

second round of FTP testing was conducted. The goal of this pilot test program was to answer 

the following question: 

Can the OBD-I/M check accurately detect evaporative system purge malfunctions and 

leaks on OBD-equipped vehicles and, once these failures are repaired and the codes 

cleared, does the OBD system respond correctly by leaving the MIL extinguished? 

Unlike the tailpipe pilot, the OBD evaporative pilot did not focus on comparing the 

OBD-I/M check to traditional I/M tests like the purge and pressure tests. The reason for this is 

because OBD-equipped vehicles with enhanced evaporative system monitoring are largely 

considered untestable using traditional I/M program evaporative system tests2 3 . In many cases, 

the intrusive nature of the traditional I/M evaporative system tests could easily compromise the 

evaporative control systems on these vehicles, which, for example, frequently have hard lines 

that cannot be crimped without causing damage to the vehicle. In other cases, the lines are 

simply inaccessible, or cannot be disconnected as required by some of the traditional evaporative 

system test procedures. 

Based upon the test results detailed in this TSD -- and given the impracticality of using 

traditional purge and pressure checks on most OBD-equipped vehicles -- EPA concluded that 

23 A notable exception is the gas cap pressure test (see section 4). Another exception is OBD-equipped vehicles that 

have been built with special evaporative system service ports. However, because such service ports are not required on these 

vehicles -- and EPA has no reliable data on how many vehicles are so equipped -- it is difficult to imagine basing a program of 

evaporative system testing upon the presumption that such ports will be generally available to facilitate testing. 

18
 
 

 

 

 

 

 

 

 

 

 

 

OBD-based evaporative emissions checks are a suitable replacement for traditional evaporative 

emission I/M tests on OBD-equipped vehicles. This conclusion is based upon the following 

observations made during the OBD evaporative system pilot: 

1) OBD evaporative system monitors appear to operate within their design specifications 

in the majority of cases. When evaporative system failures of the type found in traditional 

I/M test programs were induced, in most cases the OBD system responded correctly by 

lighting a MIL and setting an evaporative system DTC. Furthermore, once these failures 

were corrected and the codes cleared, the OBD system again responded correctly by not 

resetting the DTCs and re-lighting the MIL. 

2) The emission reductions associated with performing repairs triggered by OBD-based 

evaporative system testing appear to be substantial. In general, vehicles with evaporative 

emission DTCs and lit MILs were found to substantially exceed their FTP evaporative 

emission standards, while repaired vehicles fell well below those standards. 

3.2 Background 

In addition to monitoring components the failure of which could lead to exhaust 

emissions exceeding their FTP standards by 1.5 times the standard, OBD certification 

requirements also phase-in separate standards for monitoring the evaporative control systems on 

OBD-equipped vehicles. The first of these OBD evaporative system standards is phased in with 

the 1996 through 1999 model year and applies to those vehicles which meet the enhanced 

evaporative emission certification standards. Vehicles built to meet the enhanced evaporative 

emission standard must limit running losses to less than 0.05 gpm and hot soak/diurnal losses on 

the FTP to no more than 2.0 grams. Beginning with MY 1998, and phasing in through MY 

2006, OBD-equipped vehicles must also meet the Onboard Refueling Vapor Recovery (ORVR) 

standards, which prohibit vehicles from emitting more than 0.02 grams per gallon of fuel 

dispensed during vehicle refueling. 

To determine whether vehicles built to meet these requirements were operating as 

required in the field, EPA contracted with ATL to conduct a pilot study of OBD evaporative 

monitor effectiveness. As previously stated, the pilot testing ran from April 1999 to May 2000 

and included a mix of 30 LDVs and LDTs from MY 1996 through 20002 4 . The goal of this pilot 

was to determine whether OBD reacted correctly to evaporative system malfunctions and to the 

repair of those malfunctions, as well as to determine the degree to which either condition (i.e., 

malfunctioning vs. repaired) affected FTP evaporative emissions. This pilot did not examine the 

issue of OBD evaporative emission readiness under in-use driving conditions, nor did it address 

whether the gas cap test is a suitable supplement to OBD-I/M evaporative system testing. Those 

issues were addressed as part of EPA’s analysis of OBD test results from the Wisconsin I/M 

program and will be discussed in section 4 of this TSD. 

24 Data gathered for EPA under Work Assignments 3-12 and 0-4, SHED Tests on OBD II Evap Vehicles, EPA 

Contract No. 68-C99-241 - Automotive Testing Laboratories; 1999-2000. 

19
 
 

 

 

 

3.3 Vehicle Sampling 

3.3.1 Methodology 

Unlike the OBD tailpipe pilot, the vehicles used in the OBD evaporative pilot were not 

recruited from operating I/M lanes, and recruitment was not based upon naturally occurring, preexisting evaporative system failures. This decision was based upon prior attempts to recruit 

natural OBD-I/M failures which showed that the majority of such failures with evaporative 

system DTCs set were the result of gas caps that had not been tightened properly (this issue is 

discussed further under section 3.4.1, which addresses vehicle testing methodology). Instead, the 

majority of vehicles for the OBD evaporative pilot were recruited from fleet owners in the Mesa, 

Arizona area, including both commercial rental agencies and local auto dealerships with which 

ATL had a standing arrangement for procuring test vehicles. Only one vehicle involved in the 

pilot was a privately owned vehicle recruited from an ATL employee. Though every effort was 

made to recruit as diverse a sample as possible, no attempt was made to weight the vehicle 

sample by vehicle type and manufacturer. This was due largely to limitations resulting from the 

small sample size (less than one-sixth the size of the OBD tailpipe sample) which, in turn, was 

the result of the high cost and time requirements associated with FTP evaporative system testing, 

where a single test can take several days to complete. 

3.3.2 Results 

A complete description of the 30 vehicles participating in the OBD evaporative pilot can 

be found in Table A-1 of Appendix 6. The descriptive details identified include vehicle make, 

model, model year, mileage, engine family, evaporative system family, chassis dynamometer 

testing parameters, and whether the vehicle was designed to comply with ORVR and/or 

enhanced evaporative control standards. A snapshot of the 30-vehicle OBD evaporative test 

sample is provided below: 

Manufacturers represented: 8 

Ford (7), GM (7), Honda (3), Isuzu (1), Mazda (2), Mitsubishi (1), Nissan (4), Toyota (5) 

Model years represented: 5 

1996 (2), 1997 (1), 1998 (9), 1999 (16), 2000 (2) 

Lowest mileage: 5,259 miles Highest mileage: 116,730 miles 

Light-duty vehicles: 20 Light-duty trucks: 10 

Enhanced evap system only: 14 Enhanced evap and ORVR: 16 

Fleet vehicles: 29 Privately owned vehicles: 1 

20
 
 

 

 

 

 

As discussed in section 3.3.1, the 30-vehicle sample was not sales weighted among 

manufacturers; neither was it weighted based upon car versus truck sales. Most of the vehicles 

in the sample were of relatively low mileage, with only 5 exceeding 50,000 miles, while the 

sample average was just over 31,000 miles. As can be seen from the above manufacturer 

summary, Chrysler vehicles are not represented in the 30-vehicle sample. This is because 

Daimler-Chrysler used an alternative Federal OBD certification provision available for MY 

1996-99 vehicles which allowed manufacturers to postpone use of OBD evaporative emission 

monitors in their Federally certified vehicles until MY 2000. 

3.4 Vehicle Testing 

3.4.1 Methodology 

Prior to being accepted into the OBD evaporative pilot study, candidate vehicles were 

evaluated to ensure that there were no driveability, braking, or exhaust leak problems. Once a 

vehicle was accepted, its OBD emission control system was then checked for readiness status 

and the presence of a lit MIL and/or DTCs. Unlike the OBD tailpipe pilot, the pre-existence of 

naturally occurring system failures was not one of the criteria for participation in the OBD 

evaporative pilot. Instead, EPA opted to use induced failures. 

Induced failures were used due to the difficulty EPA had in finding MY 1996 and newer 

OBD-equipped vehicles with naturally occurring evaporative system problems, which, in turn, 

was due to the relative newness of the vehicles in question. Unlike tailpipe problems which are 

largely a function of mileage accumulation and general wear-and-tear, evaporative system 

problems tend to be a function of vehicle age, as the components of the system lose elasticity and 

become brittle and more leak-prone. Furthermore, when naturally occurring evaporative system 

DTCs were found, the vast majority turned out to be due to gas caps that had not been properly 

tightened after refueling. EPA decided to use induced evaporative system failures to more 

thoroughly investigate the effectiveness of OBD systems in detecting a wide variety of potential 

in-use failures, above and beyond loose gas caps. Table A-2 in Appendix 6 provides a vehicleby-vehicle account of the induced failures that were used in the 30-vehicle sample, the resulting 

DTCs, and the drive cycles required to satisfy the readiness criteria for both “failure” and 

“repair” sequences. Table A-2 also includes a comment field for more detail on specific vehicle 

test issues. A summary of the induced failures used in the pilot is provided in Table 10 below: 

Only one failure was induced per vehicle. The failures used were not meant to represent 

the variety of real world failures, nor were they necessarily representative of the range of excess 

emissions which results from real failures. Rather, the failures used were selected because they 

are reproducible, easy to repair, and are the sorts of failures properly functioning OBD 

evaporative system monitors should detect. Following the induced failures, vehicles were then 

given the evaporative portion of the FTP to help estimate the mass of excess evaporative 

emissions associated with these failures. 

Table 10: Summary of Induced Evaporative System Failures 

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Type of Failure Number of Instances in Sample 

Missing gas caps 3 

Loose gas caps 2 

0.040 inch leaks in gas caps, vent lines (initial OBD leak detection threshold) 11 

Disabled canister fresh air inlet 1 

Disconnected purge lines 8 

0.020 inch leaks in gas caps (stricter OBD leak threshold, begins phasing-in MY 2000) 5 

Two varieties of small orifice leaks were induced under the OBD evaporative pilot -- a 

0.040 inch leak and a more stringent 0.020 inch leak. Under California and Federal OBD 

requirements, MY 1996 and newer vehicles equipped with OBD II evaporative system monitors 

are required to detect leaks from a hole 0.040 inches in diameter or larger, and must also detect 

and identify a malfunctioning purge system2 5 . Beginning with MY 2000, LDVs and LDTs must 

begin phasing in a more stringent leak detection threshold of 0.020 inches in diameter. Under 

the OBD evaporative pilot, 5 vehicles were tested with 0.020 inch diameter leaks to examine the 

robustness of the current systems (i.e., whether or not they can detect leaks below the level 

minimally required), as well as to estimate the incremental emission impact from identifying 

vehicles which pass the 0.040 inch limit while failing the more stringent 0.020 inch limit. Gas 

caps with 0.040 and 0.020 inch diameter leaks were supplied by Stant Manufacturing 

Corporation and were built with flow tested, precision machined, square-edged orifices. 

Once a failure was induced, the impact on the vehicle’s evaporative emission system was 

verified by performing functional “pressure” and “purge” tests on the vehicle in question. These 

traditional evaporative system tests were conducted by experienced ATL laboratory technicians2 6 

who were not under the time constraints that make such testing impractical in most high volume 

I/M test lanes. For vehicles with service ports, the tests were conducted by measuring pressure 

loss and purge system vacuum through the service port. For vehicles without service ports, the 

ATL technicians used test procedures designed for pre-OBD-equipped vehicles. These pre-OBD 

test procedures consisted of measuring pressure loss by pressurizing the evaporative system from 

the fill-pipe and then monitoring the loss of pressure with time. Purge system failures were 

verified by using a roto-meter to check for the presence (or lack) of purge flow. 

After a failure condition was induced, the vehicle’s OBD computer was then reset to 

clear codes so that all monitors registered as “not ready.” The vehicles were then operated in a 

manner that would exercise the monitors and -- if the OBD system was functioning properly -- a 

DTC would be set and the MIL illuminated. Typically, “exercising the monitors” meant driving 

25 Not all MY 1996-99 vehicles are equipped with OBD evaporative system monitors. Manufacturers were allowed to 

phase-in the use of such monitors from MY 1996 through MY 1999. 

26 We stress that the repair technicians were “experienced” because many manufacturers have opposed the intrusive 

nature of EPA’s original evaporative system tests, particularly in high volume I/M lanes. Particular care was taken during this 

pilot to conduct these tests in a manner that did not adversely influence the evaporative emission results. 

22
 
 

 

 

 

 

 

 

 

the vehicle on a dynamometer prior to formal FTP testing. The only exception to this practice 

was when a dynamometer was not available, at which time “exercising the monitor” was 

achieved by operating the vehicle over a local surface street, following a route approximating the 

speed-time relation of the drive cycle used in the FTP (also known as the LA-4). Following FTP 

testing of vehicles with OBD-confirmed, induced failures, the vehicles were then repaired by 

ATL technicians. After repairs, the OBD system was again reset to clear the fault code and 

return the readiness status to “not ready.” The vehicle was then driven to exercise the monitors 

to determine if the OBD system responded correctly (i.e., by not setting a DTC or commanding 

the MIL to light). 

The FTP evaporative emission test selected for this study was the 3-day diurnal 

procedure with running loss test. An abbreviated flowchart of the test procedure for the FTP 

evaporative test is presented in Figure A-1 in Appendix 6. In general, tests were conducted in 

accordance with 40 CFR Part 86, Subpart B, as revised July 1, 19982 7 . The test fuel used in this 

pilot was indolene, as specified by the FTP. 

3.4.2 Results 

The 30-vehicle sample was divided into two groups, based upon detection threshold. The 

first is a group of 25 vehicles, 9 with purge system failures, and 16 with leaks greater than or 

equal to 0.040 inches in diameter (i.e., vehicles with induced failures within the required 

detection range of current OBD evaporative system standards). The second group consists of 5 

vehicles with induced leaks produced by a 0.020 inch diameter hole in the gas cap (i.e., vehicles 

with induced failures falling below the currently required detection threshold for OBD 

evaporative systems). The two groups were separated so as not to “penalize” the overall sample 

for vehicles in the second group which failed to find leaks more stringent than their design 

requirements. Table 11 below looks at each subset separately. 

Table 11: DTC and MIL-on Rates After Induced Failures 

DTC Registered MIL Illuminated 

25 Vehicle Sample (purge, 0.040 leaks) 22 The same 22 

5 Vehicle Sample (0.020 leaks) 3 The same 3 

Emission results for 22 vehicles repaired as part of the OBD evaporative pilot are 

summarized in Table 12 below. Only 22 of the 30-vehicle sample are included in the repair 

results summary because not all vehicles registered DTCs, and valid “repair” results were not 

accomplished for all vehicles. Vehicles without a complete set of “fail” vs. “repair” data were 

excluded from the analysis used to prepare Table 12. 

27 Some minor deviations from Subpart B were allowed during pilot testing, including the use of: 1) external fuel tank 

temperature measurement (on steel fuel tanks) as a surrogate for installing internal thermocouples, 2) the vehicle’s fuel pump to 

drain the tank instead of installing a drain(s) at the lowest point in the tank, and 3) a greater than +/- 3 degree F disagreement 

between measured and target temperatures on the running loss test for a small number of vehicles. Table A-5 in Appendix 6 lists 

all target vs. actual temperature differences observed during pilot testing (see Appendix 7 for further discussion). 

23
 
 

 

 

 

 

 

The summary results presented in Table 12 are stratified as a function of evaporative 

emission control design (i.e., whether enhanced evap or ORVR designs). Of the 22 vehicles 

included in the sample, 11 were certified to the enhanced evaporative standard and 11 were 

designed to comply with ORVR requirements. Table 12 divides the evaporative emission results 

into these strata because the design of ORVR systems (larger canisters, larger vapor lines, other 

unique components to control refueling loss) may lead to lower overall evaporative emission 

losses in the case of a leak or other malfunction. Because ORVR designs are manufacturer and 

vehicle design specific and the sample size used in the pilot was too small to be representative 

across manufacturers and models, an investigation into how and why ORVR compliant vehicles 

seem to have inherently low evaporative emissions was not performed as part of the OBD 

evaporative pilot. Nevertheless, the data in Table 12 suggest that when compared to vehicles 

designed to only meet the enhanced evaporative emission requirements, ORVR-controlled 

vehicles have significantly lower evaporative emissions, even when leaks or other malfunctions 

have been introduced into the system. 

To get an idea of the impact on the mean and standard deviations when the subsets of 

enhanced evap and ORVR vehicles are averaged together, see Table 13 below. Complete 

evaporative emission results for all 30 vehicles are presented in Table A-3 in Appendix 6, while 

FTP exhaust results for these same vehicles are summarized on a bag-by-bag basis in Table A-4. 

Table 12: Summary Statistics for 11 Enhanced Evap and 11 ORVR Vehicles 

Enhanced 

evap fails 

Enhanced 

evap repairs ORVR fails ORVR repairs 

Running Losses (gpm) x = 7.86 x = 0.02 x = 4.51 x = 0.02 

s = 7.89 s = 0.01 s = 5.29 s = 0.01 

1 hour Hot Soak Loss (grams) x = 10.74 x = 0.13 x = 2.89 x = 0.14 

s = 16.12 s = 0.08 s = 3.20 s = 0.07 

High 24 hour Diurnal Loss (grams) x = 20.83 x = 0.95 (N=10)2 8 x = 12.31 x = 0.87 

s = 17.77 s = 0.87 s = 12.00 s = 0.51 

x = mean; s = standard deviation 

Table 13: Average Emission Reductions From Sample of 22 Repaired Vehicles 

Running Losses (gpm) x = 6.17 s = 6.78 

1 hour Hot Soak Loss (grams) x = 6.68 s = 12.04 

High 24 hour Diurnal Loss (grams) x = 14.18 (N=21)2 9 s = 14.54 

In addition to the summary results presented in Tables 12 - 13 above, EPA wishes to 

highlight the following findings made as a result of the OBD evaporative pilot study: 

28 One of the 11 enhanced evap vehicles had to be returned to its owner prior to post-repair evaporative system testing. 

29 See explanation in footnote 28 above. 

24
 
 

 

 

 

 

 

 

1) 88% of OBD-equipped vehicles (22 of 25) set DTCs and lit MILs when 

evaporative system failure conditions were induced and subsequently showed no 

DTCs and MILs when the vehicles were repaired. This suggests that OBD 

evaporative system monitors are working as designed in the vast majority of 

cases. EPA considers these results impressive, compared to the existing purge 

and fill-neck pressure tests, which both suffer from relatively low testability rates 

-- approximately 70% for pre-OBD-equipped vehicles and less than 15% for 

OBD-equipped vehicles. 

2) Of the 3 vehicles that did not light a MIL or set a DTC during ATL testing, 2 were 

Mazda 626s, which represents 100% of that manufacturer’s fraction of the 30-vehicle 

sample. To see whether there was a possible design problem with this particular make 

and model, EPA procured two “sister” vehicles in Ann Arbor, and found them to be 

functioning properly. EPA is pursuing a resolution regarding the third vehicle’s results 

with the manufacturer. 

3) 60% of OBD-equipped vehicles tested (3 of 5) identified a 0.020 inch diameter leak 

(i.e., a leak below the required leak detection threshold for the OBD-equipped vehicles in 

the sample) by setting a DTC and lighting the MIL. This suggests that the majority of 

OBD systems are quite robust and have leak detection capability well below the 

minimum requirement. 

4) Three OBD-equipped vehicles which set MILs for evaporative system problems 

(different from the 3 of 5 listed in item # 3 above) produced FTP evaporative emissions 

less than half the levels of the enhanced evaporative emission standards. This suggests 

that “maintenance” problems are being identified by OBD even though they result in 

emission levels below FTP standards. 

5) 95% of repaired OBD-equipped vehicles (21 of 22) had FTP-measured running loss 

emissions that were actually below the certification standard for enhanced running losses. 

95% of repaired OBD-equipped vehicles (20 of 21) had FTP-measured diurnal plus hot 

soak emissions that were below the certification standards for those categories of 

evaporative emissions. The running loss and diurnal plus hot soak emissions for ORVR 

vehicles with induced failures averaged approximately half the levels measured for 

comparable, enhanced evap-only vehicles. 

6) The average emission reductions for repairing OBD-identified DTCs is substantial: 

6.17 gpm for the running loss test, 6.7 g for the hot soak test, and 14.2 g for the high 24 

hour result for the diurnal loss test. 

3.5 Conclusions 

In the majority of cases, OBD evaporative emission monitors appear to be operating as 

designed, and, in some cases, better than required. This conclusion is based upon an admittedly 

25
 
 

 

 

 

 

 

 

 

small sample of OBD-equipped vehicles with induced failures specifically aimed at triggering 

the evaporative system monitors. Nevertheless, the majority of OBD-equipped vehicles in the 

test sample correctly set evaporative DTCs and lit the MIL when evaporative system failures 

were induced, while also successfully showing no MILs or DTCs when those failures were 

removed. An analysis of the FTP mass emissions data before and after these induced failures 

suggests that the emission reductions attributable to OBD-triggered evaporative system repairs is 

substantial, with pre-repair vehicles registering evaporative emissions well above the applicable 

FTP standards and post-repair vehicles having evaporative emissions well below those standards. 

Based upon these observations -- and given the impracticality of using pre-OBD-style purge and 

pressure checks on most OBD-equipped vehicles -- EPA believes that OBD-I/M evaporative 

emissions checks are a suitable replacement for the traditional purge and fill-neck pressure tests 

for MY 1996 and newer, OBD-equipped vehicles. 

Neverthless, EPA still recommends that states continue to perform gas cap pressure tests 

on OBD-equipped vehicles. Unlike other, traditional evaporative system tests, the gas cap test 

does not suffer from the material composition and accessibility problems that make many OBDequipped vehicles untestable using the purge and fill-neck pressure tests. Furthermore, the 

failure threshold for the gas cap pressure test is more stringent than even the most stringent 

OBD-based evaporative emission standards. As will be shown in section 4, which details EPA’s 

analysis of Wisconsin’s operating OBD-I/M program data, EPA believes that there is real-world 

data to suggest that additional evaporative system failures can be identified by performing a 

separate gas cap pressure test in conjunction with the OBD-I/M check (see Table 19, “Gas Cap 

vs. OBD Evaporative System Failure Rates” later in this document). 

4.0 OBD-I/M Pilot 3: Analyzing the Wisconsin OBD-I/M Program Experience 

4.1 Summary of Goals and Conclusions 

The last of the three OBD-I/M pilot studies was aimed at identifying the real-world 

implementation issues associated with OBD-I/M testing and was conducted using data gathered 

from the Wisconsin enhanced I/M test lanes, where OBD checks are being implemented 

voluntarily by the state. The analysis of Wisconsin’s operating program data for OBD-equipped 

vehicles was conducted in two stages, the first performed by Sierra Research in 1998 under 

contract to EPA and the second in 1999-2000, performed by EPA staff from the Office of 

Transportation and Air Quality (OTAQ) in Ann Arbor, Michigan. 

Although the original focus of the Sierra study was intended to be broader, flaws in 

Wisconsin’s I/M contractor’s OBD hardware and software limited the scope of the analysis to 

identifying physical aspects of the OBD-I/M testing process that could lead to implementation 

difficulties. Specifically, the Sierra study provided an estimate of the time needed to perform a 

typical OBD-I/M inspection (on average, about 31 seconds) and also identified atypical data link 

connector (DLC) location as a potential bottleneck in high-volume I/M test lanes. In response to 

this latter issue, EPA has developed a database of DLC locations based upon the Wisconsin data 

and manufacturer-supplied information. Electronic copies of this database are available on 

26
 
 

 

 

 

 

 

 

 

 

 

EPA’s web site at http:\\www.epa.gov\OMSWWW. EPA has found that the development of this 

database and increased inspector experience has eliminated DLC location as a problem area in 

the Wisconsin program. While a period of introductory learning will be necessary, we do not 

believe that DLC location will be a significant problem for future, OBD-based I/M efforts. 

Separate from the Sierra Research analysis, EPA looked at data from Wisconsin’s I/M 

program for the last eight months of 1999, by which time the OBD software and hardware 

problems mentioned above had been corrected. The program data EPA analyzed included 

IM240, gas cap, and OBD MIL illumination and readiness data for over 116,000 MY 1996 and 

newer vehicles. 

Using the above database, EPA compared evaporative system failure rates for vehicles 

based upon the OBD evaporative system test and the separate gas cap check and found that the 

gas cap test identified significantly more evaporative system leaks than were identified based 

upon the OBD evaporative system monitors alone. EPA believes this demonstrates the 

complementary nature of these two tests -- not an unanticipated conclusion, given the different 

standards and stringencies involved. We believe that these findings support our recommendation 

that states continue gas cap evaporative system testing on OBD-equipped vehicles in conjunction 

with OBD-I/M testing (as mentioned earlier, in section 3 of this TSD). 

In analyzing Wisconsin’s OBD-I/M data, EPA also looked at MIL illumination and 

monitor readiness results and concluded that there is a small fraction of vehicles that arrive for 

testing with one or more of their OBD readiness codes unset, although the problem seems largely 

limited to the earliest of the OBD-equipped vehicles (i.e., MY 1996). Looking at the raw data, 

EPA found a 5.8% not-ready rate among MY 1996 vehicles. However, when we excluded 

vehicles for which corrective measures are being taken by the manufacturers, the not-ready rate 

for MY 1996 dropped to roughly 3%. By MY 1998, the OBD not-ready rate dropped even 

further -- to below 1%. EPA believes that offering states the ability to waive vehicles with one 

or two unset readiness codes instead of rejecting them (as currently required) will go a long way 

toward eliminating vehicle readiness as an obstacle to smooth implementation. 

Because the two-staged analysis of Wisconsin’s OBD-I/M data has three separate points 

of focus – DLC location, vehicle readiness, and the relative effectiveness of the gas cap test – 

each will be dealt with individually below, identified by focus. 

4.2 DLC Location 

4.2.1 Background 

In 1998, EPA contracted with Sierra Research to gather information on approximately 

2,500 OBD-equipped vehicles upon arrival at the I/M test lanes in Wisconsin3 0 . Parallel IM240 

and OBD-I/M testing was conducted in the Wisconsin test lanes by Envirotest Systems 

30 Under Purchase Order No. 7CS124NTSA, “Status of OBD Systems Upon Arrival at I/M Lanes.” Report No. 

SR98-10-02, “Summary of Test Results from Wisconsin EPA OBD Project,” October 16, 1998. 

27
 
 

 

 

 

 

 

 

 

 

 

Corporation (under subcontract to Sierra Research, Inc.). The original purpose of the study was 

to conduct testing of OBD systems on MY 1996 and newer vehicles, with the intent to: 

•	 Gain practical experience in conducting OBD-I/M tests; 

•	 Use this experience to develop guidance on how to perform OBD-I/M tests properly; 

•	 Estimate the average time required to perform an OBD-I/M test; 

•	 Determine the frequency of readiness for each OBD system monitor; 

•	 Determine the reliability of the MIL as an identifier of vehicles likely to fail other I/M 

tests; 

•	 Determine the frequency and nature of DTCs stored in OBD computers; and 

•	 Identify any problems among OBD-equipped vehicles that could interfere with proper 

testing, including those specific to particular vehicle models. 

4.2.2 Results 

Between May 20, 1998, through July 25, 1998, 2,583 paired OBD/IM240 test records 

were collected. Only initial tests were used because Wisconsin does not currently fail vehicles 

on the basis of the OBD-I/M check3 1 . Information was gathered concerning test time, OBD 

readiness, DTC and/or MIL frequency, etc. However, due to problems with the OBD software 

and hardware used by Wisconsin’s testing contractor at the time of the Sierra study, no useful 

information was gathered by Sierra concerning OBD readiness or DTC and/or MIL-on rates. 

Nevertheless, useful information was gathered concerning test time and the general ability of 

inspectors to locate difficult-too-find DLCs. Table 14 below provides a summary of the data 

gathered, divided by make, model, and vehicle type. 

As can be seen from Table 14, the average OBD-I/M test time was roughly 31 seconds, 

including the time to locate the OBD connector, connect to the system, interrogate it, and 

download the resulting information3 2 . Care should be taken in interpreting the test time 

estimates, however, due to possible variance in how inspectors conducted the test. Envirotest’s 

inspectors were instructed to locate the OBD connector as soon as they were prompted to do so 

31 While Wisconsin does not fail vehicles on the basis of OBD (yet), the State does fail vehicle based upon their IM240 

results. The Sierra study focused on initial tests only to avoid double counting vehicles which failed their initial IM240 and then 

returned for a retest. 

32 It should be noted that these test time calculations do not include the time needed to record vehicle information, such 

as VIN, license plate number, etc. Such information was gathered as part of the overall testing process, which also included 

performance of an IM240, as previously discussed. No vehicle in the Wisconsin program received just the OBD test. The test 

times discussed here, therefore, reflect only the time spent conducting the OBD-I/M portion of the overall test process. 

28
 
 

 

estimates, however, due to possible variance in how inspectors conducted the test. Envirotest’s 

inspectors were instructed to locate the OBD connector as soon as they were prompted to do so 

by the test system. The system tracked the time from when the inspector prompt appeared to 

when the connection to the OBD system was established. However, it is obvious from the short 

connect times recorded for some vehicles (e.g., 1-2 seconds) that certain inspectors were locating 

the connector in advance of the prompt. Actual connect times (and thus overall test time as well) 

may therefore be slightly longer than the data in Table 14 suggest. However, given how the test 

is structured, it is believed that this would add only about five seconds at most to some of the 

recorded OBD-I/M test times. 

The time it takes to locate the DLC is a relevant variable in assessing the time it takes to 

perform an OBD-I/M inspection because DLC location varies from manufacturer to 

manufacturer, and from model to model. Attempts to standardize DLC location are reflected in 

the Society of Automotive Engineers (SAE) Recommended Practices J1962, which specifies the 

following with regard to DLC location: 

3.1 Consistency of Location - The vehicle connector shall be located in the passenger 

compartment in the area bounded by the driver’s end of the instrument panel to 300 mm 

beyond the vehicle centerline, attached to the instrument panel, and accessible from the 

driver’s seat. The preferred location is between the steering column and the vehicle 

centerline. The vehicle connector shall be mounted to facilitate mating and unmating. 

3.2 Ease of Access - Access to the vehicle connector shall not require a tool for the 

removal of an instrument panel cover, connector cover, or any barriers. The vehicle 

connector shall be fastened and located so as to permit a one-handed/blind insertion of 

the mating test equipment connector. 

3.3. Visibility - The vehicle connector should be out of the occupant’s (front and rear 

seat) normal line of sight but easily visible to a “crouched” technician. 

Even with this guidance, however, vehicle manufacturers have been anything but consistent with 

regard to where they place the DLC. 

29
 
 

 

Table 14: Summary of Wisconsin Data, 5/20/98 to 7/25/98 

Make 

Acura 

Audi 

BMW 

Buick 

Cadillac 

Type 

LDGV 

LDGV 

LDGV 

LDGV 

LDGV 

Number Tested 

19 

3 

23 

117 

41 

Connect Time (sec) 

28.9 

6.0 

25.3 

12.8 

23.5 

Comm Time (sec) 

13.3 

14.0 

14.7 

12.8 

17.3 

Total OBD Time (sec) 

42.2 

20.0 

40.0 

25.6 

40.8 

Chevrolet 

Chrysler 

Datsun 

Dodge 

Eagle 

LDGV 

LDGT1 

LDGT2 

HDGT 

LDGV 

LDGT1 

LDGV 

LDGT1 

LDGV 

LDGT1 

LDGT2 

HDGT 

LDGV 

182 

101 

75 

6 

42 

25 

105 

17 

88 

115 

30 

6 

3 

14.8 

12.4 

12.6 

24.0 

14.9 

11.6 

15.9 

29.3 

16.5 

10.7 

14.4 

19.5 

18.3 

15.0 

15.8 

13.9 

5.8 

18.0 

18.2 

16.7 

18.6 

15.6 

16.0 

16.1 

12.7 

16.0 

29.8 

28.2 

26.5 

29.8 

32.9 

29.8 

32.6 

47.9 

32.0 

26.7 

30.5 

32.2 

34.3 

Ford 

Geo 

LDGV 

LDGT1 

LDGT2 

HDGT 

LDGV 

LDGT1 

144 

117 

44 

10 

23 

2 

13.5 

18.6 

16.6 

36.7 

16.0 

5.5 

13.7 

15.7 

13.0 

13.2 

14.3 

17.0 

27.2 

34.3 

29.7 

48.9 

30.3 

22.5 

GMC 

Honda 

Hyundai 

Infiniti 

LDGT1 

LDGT2 

HDGT 

LDGV 

LDGT1 

LDGV 

LDGV 

21 

17 

5 

127 

20 

4 

2 

11.8 

10.7 

19.4 

12.8 

33.5 

2.7 

1.5 

14.4 

15.2 

13.8 

16.9 

15.9 

15.7 

16.5 

26.2 

25.9 

33.2 

29.7 

49.3 

18.3 

18.0 

Isuzu 

Jaguar 

Jeep 

Lexus 

LDGT1 

LDGT2 

LDGV 

LDGT1 

LDGV 

15 

2 

4 

80 

14 

9.8 

24.5 

21.3 

17.1 

13.2 

13.8 

15.0 

14.3 

21.3 

18.1 

23.6 

39.5 

35.5 

38.4 

31.3 

Lincoln 

Landrover 

LDGV 

LDGT2 

26 

5 

17.3 

19.5 

13.2 

14.0 

30.5 

33.5 

Mazda 

Mercury 

Mercedes 

LDGV 

LDGT1 

LDGV 

LDGT1 

All 

33 

2 

98 

33 

15 

17.1 

23.0 

15.3 

11.7 

22.2 

14.1 

27.0 

13.4 

17.1 

15.1 

31.2 

50.0 

28.6 

28.8 

37.3 

Mitsubishi 

Oldsmobile 

Other 

LDGV 

LDGT1 

LDGV 

LDGT1 

LDGT1 

14 

3 

56 

11 

1 

19.7 

32.3 

18.1 

16.0 

40.0 

15.9 

14.3 

14.7 

14.0 

12.0 

35.6 

46.7 

32.8 

30.0 

52.0 

Plymouth 

Pontiac 

Porsche 

Saab 

Sterling 

Subaru 

Suzuki 

LDGV 

LDGT1 

LDGV 

LDGT1 

LDGV 

LDGV 

LDGV 

LDGV 

LDGT1 

32 

48 

168 

13 

1 

7 

61 

9 

2 

14.2 

9.6 

15.8 

8.9 

80.0 

20.6 

13.5 

53.7 

31.5 

16.1 

15.4 

14.2 

14.8 

14.0 

14.6 

17.8 

15.4 

11.5 

30.3 

25.0 

29.9 

23.8 

94.0 

35.1 

31.3 

69.1 

43.0 

Toyota 

Volkswagen 

Volvo 

LDGV 

LDGT1 

LDGT2 

LDGV 

LDGV 

199 

45 

4 

45 

5 

12.2 

22.0 

32.0 

18.2 

14.2 

15.7 

14.8 

18.0 

18.4 

15.0 

27.9 

36.8 

50.0 

36.6 

29.2 

TOTAL 2,583 15.6 15.6 31.2

30
 
 

 

Table 15: DLC Location Problems in the Sierra Wisconsin Data 

Make Total in sample # DLC location problem 

% DLC location problem in 

sample of make 

Acura 

Audi 

BMW 

Buick 

Cadillac 

Chevrolet 

Chrysler 

Datsun 

Dodge 

Ford 

Geo 

GMC 

Honda 

Hyundai 

Isuzu 

Lexus 

Mazda 

Mercury 

Mercedes 

Mitsubishi 

Oldsmobile 

Pontiac 

Subaru 

Suzuki 

Toyota 

Volkswagen 

Volvo 

19 

3 

23 

117 

41 

364 

67 

122 

239 

315 

25 

43 

147 

4 

17 

14 

35 

131 

15 

17 

67 

179 

9 

2 

248 

45 

5 

14 

3 

18 

2 

3 

2 

1 

1 

3 

7 

1 

1 

69 

1 

2 

1 

1 

3 

12 

7 

3 

1 

1 

2 

20 

25 

4 

11.8% 

100% 

78.3% 

1.7% 

7.3% 

0.6% 

1.5% 

0.8% 

1.3% 

2.2% 

4% 

2.3 

46.9% 

25% 

11.8% 

7.1% 

2.9% 

2.3% 

80% 

41.2% 

4.5% 

0.6% 

11.1% 

100% 

8.1% 

55.6% 

80% 

TOTAL 2,583 208 8.1% 

Ultimately, OBD-I/M test times were found to be highly dependent on the ease with 

which the inspector located the DLC, which was itself found to vary greatly among the various 

makes and models included in the Sierra Research study. During the first phase of the 

Wisconsin analysis, Sierra Research found that it took considerably longer to locate the DLC on 

some makes and models than it did for others. Apparently, despite attempts to standardize DLC 

location, some manufacturers have interpreted SAE J1962 more broadly than originally 

anticipated. In fact, many vehicles were identified by inspectors as “untestable” because they 

could not locate the DLC in a timely manner. Out of the 2,583 vehicles involved in the test 

31
 
 

 

 

 

 

 

program, 208 or 8.1% were identified as “untestable,” largely because the inspector was unable 

to locate the connector within a time period that was commensurate with the throughput demands 

of the test network. In fact, DLC location is not so much an issue of “testability” as it is an issue 

of throughput. Table 15 above provides a breakdown of these so-called “untestable” vehicles by 

manufacturer. 

Designating a given vehicle as “untestable” proved to be highly subjective. While nearly 

all of the test records represented in Table 15 contain a connector location of “99” (signifying a 

location other than somewhere under the front dashboard), many of the records also contain 

inspector comments indicating they were either unable to find the connector or it was found in an 

abnormal location. Some inspectors were apparently able to test them, while others could not 

locate the connectors, and still others could locate the connectors but indicated it was too much 

effort to do so. For example, on the BMW 318i a cover panel must be removed with a 

screwdriver before the OBD connector can be accessed. 

4.2.3 Conclusions 

In general, the OBD connector was located by one or more inspectors on a very high 

fraction of test vehicles. However, without further efforts, the difficulty in accessing some of the 

connectors could have a significant impact on the future success of large-scale OBD-I/M testing. 

Accessibility time is particularly an issue in a high-volume test-only I/M environment. 

Inspectors are under continuous pressure from both motorists and management to be as fast and 

efficient as possible in completing required inspection procedures. Any connectors that take 

over 15-30 seconds to locate and access are a problem in this environment. While it is expected 

that inspectors will quickly learn the abnormal DLC locations for higher volume makes and 

models (e.g., behind the ash tray on Honda passenger cars), EPA believes that the potential for 

start-up 

problems is nevertheless significant. To address this issue, EPA has therefore developed a 

database of atypical DLC locations based upon the Wisconsin data, as well as manufacturersupplied information. This database is available electronically at: 

www.epa.gov/otaq/regs/im/obd/obd-im.htm 

4.3 Vehicle Readiness 

4.3.1 Background 

The OBD system monitors the status of up to 11 emission control related subsystems by 

performing either continuous or periodic functional tests of specific components and vehicle 

conditions. The first three testing categories – misfire, fuel trim, and comprehensive components 

– are continuous, while the remaining eight only run after a certain set of conditions has been 

met. The algorithms for running these eight, periodic monitors are confidential to each 

manufacturer and involve such things as ambient temperature as well as driving times and 

conditions. Most vehicles will have at least five of the eight remaining monitors (catalyst, 

evaporative system, oxygen sensor, heated oxygen sensor, and exhaust gas recirculation or EGR 

system) while the remaining three (air conditioning, secondary air, and heated catalyst) are not 

32
 
 

 

 

 

 

 

necessarily applicable to all vehicles. When a vehicle is scanned at an OBD-I/M test site, these 

monitors can appear as either “ready” (meaning the test in question has been run), “not ready” 

(meaning the test has not been run yet), or “not applicable” (meaning the vehicle is not equipped 

with the components in question). 

Current Federal regulations for OBD-I/M testing require that I/M programs reject from 

further testing any MY 1996 or newer OBD-equipped vehicle that is found to have one or more 

unset readiness codes. It is important to note that “rejection” is distinct from “failure.” In the 

context of the OBD-I/M check, rejection is triggered by a vehicle’s readiness status while failure 

is related to the presence of DTCs that command the MIL to be lit. If DTCs are present and the 

MIL is commanded on, the vehicle is failed, the initial test process is considered complete, and 

an official test report is generated. If, on the other hand, unset readiness codes are present, the 

vehicle is rejected and the test process is aborted until such time as all readiness codes are set. 

The reason vehicles with unset readiness codes are rejected but not failed is because an 

unset readiness code is not necessarily an indication of an emission problem. Rather, it is an 

indication that certain monitor(s) that are intended to determine whether or not there may be an 

emission problem have not been run to evaluate the system. In the case of rejection, the issue of 

whether or not the vehicle requires repairs is deferred until the readiness code(s) have been set 

and the monitor(s) run. 

There are many reasons why a readiness code may not be set when an OBD-equipped 

vehicle arrives at the I/M test site – some of them wholly legitimate and beyond the control of 

the motorist. For example, if the battery is disconnected during servicing or the monitors are 

turned off with a scan tool, it takes a varying amount of time for the monitors to reset, and some 

may still not be ready when the vehicle shows up for its I/M inspection. It is also possible that 

the battery was disconnected on purpose in an attempt to fraudulently extinguish the MIL and 

clear DTCs prior to OBD-I/M testing. While it is true that disconnecting the battery will 

temporarily clear any DTCs that are present, many of these will be quickly reset (in particular, 

the continuous monitors discussed above). In fact, readiness codes were developed specifically 

to prevent vehicle owners from evading the OBD-I/M test by disconnecting their batteries just 

prior to testing. In many cases, exercising the monitors to set a readiness code may be as simple 

as operating the vehicle on a dynamometer or on the highway for a certain amount of time, while 

in other cases, readiness is more difficult to establish because of design issues with certain makes 

and models of vehicles. 

To determine the extent to which vehicles may be appearing for their OBD-I/M check 

with unset readiness codes in the real world, EPA looked at OBD readiness data from 

Wisconsin’s I/M program for the last eight months of 1999. The program data EPA analyzed 

included IM240, gas cap, and OBD MIL illumination and readiness data for over 116,000 MY 

1996 and newer vehicles. The data was analyzed to determine the size of the readiness problem, 

the number of model years affected, and the approximate percentage of vehicles that would be 

rejected under a variety of possible readiness criteria. EPA also looked at the frequency of MIL 

illumination across model years and vehicle types, and compared the relative failure rates of the 

OBD-I/M check versus lane-based IM240s. 

4.3.2 Results 

33
 
 

 

 

 

 

 

 

 

 

4.3.2.1 Readiness 

Since August 1998, Envirotest Systems Corporation (the I/M contractor in Wisconsin) 

has been sending EPA staff OBD-I/M check and IM240 test data collected on MY 1996 and 

newer vehicles coming through the Wisconsin test lanes. The data provided by Envirotest 

included vehicle identification numbers (VIN), and IM240, OBD-I/M, and gas cap test results 

(for a full list of the 40 data fields included in the Wisconsin data, see Appendix 8). Because of 

the OBD software and hardware problems discussed earlier, EPA limited its analysis to data 

gathered beginning with May 1999, by which time the software and hardware issues had been 

resolved. 

Table 16 below provides the average mileage accumulation for MY 1996 and 1998 

vehicles from the Wisconsin data set. Because Wisconsin did not include odometer data until 

recently and only tests vehicles every other year, the data EPA has available for MY 1997 

vehicles does not include odometer readings. However, because the data we have for MY 1998 

and 1997 represents vehicles that are being tested at the same age (i.e., when they are one year 

old) we can assume that the average mileage accumulation for MY 1997 vehicles at the time of 

their first test is similar to that of MY 1998 vehicles at the time of their first test (i.e., between 

20,000 to 22,000 miles, depending upon vehicle class). 

Because of the different emission standards for LDVs versus LDTs, these vehicle classes 

were analyzed separately. Looking at the three model years and two vehicle classes represented 

in the Wisconsin data therefore forms six vehicle categories: 1996, 1997, and 1998 LDVs and 

LDTs. There is one caveat concerning these groupings, however. Because Wisconsin used the 

same IM240 cutpoints for some light trucks as it did passenger cars (mostly four cylinder S10s, 

Rangers, etc.) these LDTs were listed as LDVs for these analyses (see Appendix 8 for a 

discussion of EPA’s analysis methodology). 

Table 16: Average Mileage Accumulation, by Model Year and Vehicle Type 

Vehicle class 1996 1998 

LDVs 45,385 20,745 

LDTs 51,018 22,962 

Table 17 below presents the “not ready” status for MY 1996-98 LDVs and LDTs in the 

Wisconsin data set. Note that the majority of the “not ready” vehicles are MY 1996 LDVs 

(6.9%) and that the majority (77%) of all “not ready” MY 1996 LDVs were not ready for the 

catalyst monitor, while MY 1998 LDVs were more frequently “not ready” for the evaporative 

system monitor. Note further that by MY 1998, the “not ready” rate for LDVs dropped over 

five-fold – from 6.9% to 1.3% – while the overall “not ready” rate for MY 1996 vehicles (5.8%) 

dropped more than four-fold by MY 1998 – to 1.4 %. 

We can speculate that this difference in “not ready” rates among the three model years 

reflects a maturation curve for OBD technology, with the systems improving as manufacturers 

gained experience with what did and did not work in the real world. By the same token, 

34
 
 

 

 

 

 

however, it is also possible that the lower occurrence of readiness problems among the newer 

model years could be the result of differences in relative age and/or mileage accumulation. A 

test of this latter hypothesis would be to look at vehicles from different model years when they 

are at the same age and have accumulated comparable mileage. As discussed above, the 

Wisconsin data includes test results from MY 1997 and MY 1998 vehicles that were receiving 

their first tests on their one-year anniversary of purchase. Unfortunately, as we also discussed 

above, EPA does not have mileage accumulation data for MY 1997. Nevertheless, in the 

absence of any compelling reason to assume that MY 1997 vehicles were driven more or less 

than their MY 1998 counterparts in their first year of operation, we believe it is reasonable to 

assume that MY 1997 and MY 1998 vehicles exhibited comparable, accumulated mileage. The 

fact that the data in Table 17 still shows a significant decline in “not ready” rate from MY 1997 

to MY 1998 – from an overall average of 2.3% to 1.4% – suggests that manufacturer learning 

curve is at least a likely explanation for the significant trend toward improvement in observed 

“not ready” rates. 

As discussed in the background section above, some instances of vehicle unreadiness are 

due to vehicle design issues which EPA and CARB are still working with vehicle manufacturers 

to resolve. In the interim, it does not seem right to penalize motorists for something that is 

beyond their control. One logical solution is to allow states the flexibility (and the discretion) to 

not reject certain vehicles if the only problem is that they have unset readiness codes. The 

natural question then is, how do you allow these exemptions from the readiness criteria without 

opening the door to motorist fraud? In discussing this issue with the states and other interested 

OBD and I/M stakeholders, EPA concluded that the key is to limit the use of readiness 

exemptions – first, by model year, and secondarily, by the number (and possibly category) of 

unset readiness codes allowed. 

Table 17: “Not Ready” (NR) Status for MY 1996-98 

Total 

Tested 

Not Ready 

(NR) One NR Two NR Catalyst Evap O2 Heated O2 EGR valve 

96 LDV 27,313 1,873 1,155 884 1,435 475 826 880 1,041 

% 6.9% 4.2% 3.2% 5.3% 1.7% 3.0% 3.2% 3.8% 

96 LDT 16,423 651 169 64 471 184 74 186 72 

% 4.0% 1.0% 0.4% 2.9% 1.1% 0.5% 1.1% 0.4% 

96 Total 43,736 2,524 1,324 948 1,906 659 900 1,064 1,113 

% 5.8% 3.0% 2.2% 4.4% 1.5% 2.1% 2.4% 2.5% 

97 LDV 14,946 360 58 30 87 209 38 102 33 

% 2.4% 0.4% 0.2% 0.6% 1.4% 0.3% 0.7% 0.2% 

97 LDT 7,656 171 34 14 88 77 11 31 18 

% 2.2% 0.4% 0.2% 1.1% 1.0% 0.1% 0.4% 0.2% 

97 Total 22,602 531 92 44 175 286 49 133 51 

% 2.3% 0.4% 0.2% 0.8% 1.3% 0.2% 0.6% 0.2% 

98 LDV 27,615 361 101 61 105 287 59 61 55 

% 1.3% 0.4% 0.2% 0.4% 1.0% 0.2% 0.2% 0.2% 

98 LDT 22,716 350 69 32 221 182 32 55 17 

% 1.5% 0.3% 0.1% 1.0% 0.8% 0.1% 0.2% 0.1% 

98 Total 50,331 711 170 93 326 469 91 116 72 

% 1.4% 0.3% 0.2% 0.6% 0.9% 0.2% 0.2% 0.1% 

TOTAL 116,669 3,766 1,586 1,085 2,407 1,414 1,040 1,313 1,236 

% 3.2% 1.4% 0.9% 2.1% 1.2% 0.9% 1.1% 1.1% 

35
 
 

 

 

 

Determining the optimum combination of limiting factors, however, required real-world 

data. Therefore, in performing its analysis of the Wisconsin data, EPA also looked at the impact 

of adjusting “not ready” rates based upon a variety of possible readiness waiver scenarios. For 

example, does it make more sense to exempt vehicles based upon a certain number of “not 

ready” codes? Or would it be better to limit the exemptions to vehicles presenting as “not ready” 

for specific OBD monitors? In Table 17 above, the column headed “One NR” reflects the “not 

ready” rate by model year and vehicle type adjusted to reflect a waiver of the “not ready” 

rejection requirement if only one monitor is listed as “not ready.” The column headed “Two 

NR” reflects a similar adjustment of the “not ready” rate, but this time assuming a waiver of the 

rejection requirement if up to two monitors are listed as “not ready.” Table 17 also breaks out 

the readiness status of the vehicles in the Wisconsin data by monitor. 

Table 17 shows that if any one monitor is allowed to be “not ready” the overall rejection 

rate among MY 1996-98 vehicles goes from 3.2% to 1.4%. If exemptions are allowed for 

vehicles with up to two unset readiness codes, the overall rejection rate goes down even further – 

to 0.9%. Because Wisconsin did not fail vehicles on the basis of the OBD-I/M check – which 

was being conducted on a purely advisory basis at the time this data was collected – vehicles 

were also not being rejected for unset readiness codes. As a result, no attempt was made at the 

test lanes to exercise these monitors prior to continuing the test. EPA therefore believes that the 

relative “not ready” rates reflected in Table 17 represent the worst-case scenario for these model 

years, and that the frequency of unresolved “not ready” codes in a fully implemented OBD-I/M 

program will be even lower. 

4.3.2.2 MIL-on and IM240 Failure Rates 

Table 18 below compares the relative failure rates for the OBD-I/M check versus the 

IM240 test observed in Wisconsin, and the degree to which the test results overlap. As can be 

seen from the data, the OBD-I/M check almost always fails slightly more vehicles than does the 

IM240 (MY 1998 LDVs are the only exception). There are several obvious reasons for the 

marginal difference in failure rate between these two tests: 

1)	 The cutpoints for OBD are more stringent than the IM240 (i.e., 1.5 vs. 2 times the 

certification standard); 

2)	 The IM240 only monitors vehicle performance for approximately 4 minutes over 

a limited number of operating modes, while OBD performs ongoing monitoring 

of vehicle performance over the full range of operating conditions; and 

3)	 OBD monitors individual systems and components for any sign of degradation 

thus allowing it to identify necessary maintenance prior to the vehicle’s producing 

high emissions, while the IM240 can only identify vehicles which have already 

become high emitters (see discussion on “OBD and Preventative Maintenance” in 

section 2). 

36
 
 

 

 

 

 

 

Table 18 also makes clear that the agreement between IM240 and the OBD-I/M check is 

exceedingly low for all model years and vehicle types. What is not clear from this data is which 

of the two tests is more beneficial to the environment. Traditionally, relative failure rate has 

been used as a crude indicator of test effectiveness in I/M programs, with the assumption being 

that the more vehicles that are failed, the more emission reductions are being achieved. Using 

simple, relative failure rates, Table 18 suggests that the OBD-I/M check is the more effective 

test, environmentally, because it has the greater overall failure rate. 

When we use gross failure rate as our indicator of environmental effectiveness, however, 

we are ignoring one very important factor: false failures. After all, any test can be made to have 

a high failure rate – up to and including 100% – if one just makes the cutpoints tight enough and 

does not choose to worry about false failures and their impact on overall program acceptance. 

And, as indicated above, the OBD-I/M check does have more rigorous cutpoints than does the 

IM240. However, as suggested by the data presented in section 2 of this TSD, substantial 

evidence suggests that lane-based IM240s can produce false failure rates at least as high as that 

resulting from OBD-I/M testing on OBD-equipped vehicles, due to improper preconditioning, 

infrequent and/or inadequate quality assurance, etc. Conversely, section 2 also suggests that the 

vast majority of OBD-identified failures did trigger needed repairs and/or maintenance. Finally, 

manufacturers have an incentive to minimize MIL illumination when no detectable problem 

exists. Therefore, we expect false MIL illumination to be a decreasing problem. Taken together, 

these findings suggest that comparing the two tests – the OBD-I/M check versus IM240 – the 

OBD-I/M check will have no higher and perhaps less of a false failure rate than the IM240. The 

OBD-I/M check may therefore have a marginally higher absolute failure rate and ability to 

identify problem vehicles when compared to the IM240. 

Table 18: OBD vs. IM240 Fails 

MY/Class Total Tested 

OBD Fail 

(number) 

OBD Fail 

(percent) 

IM240 Fail 

(number) 

IM240 Fail 

(percent) 

Failed Both 

(number) 

Failed Both 

(percent) 

1996 LDV 27,313 645 2.4% 569 2.1% 59 0.2% 

1996 LDT 16,422 436 2.7% 383 2.3% 100 0.6% 

1996 Total 43,735 1,081 2.5% 952 2.2% 159 0.4% 

1997 LDV 14,944 91 0.6% 71 0.5% 7 0.2% 

1997 LDT 7,656 66 0.9% 51 0.7% 0 0.0% 

1997 Total 22,600 157 0.7% 122 0.5% 7 0.0% 

1998 LDV 27,616 118 0.4% 223 0.8% 7 0.0% 

1998 LDT 22,716 123 0.5% 47 0.2% 0 0.0% 

1998 Total 50,332 241 0.5% 270 0.5% 7 0.0% 

4.3.3 Conclusions 

37
 
 

 

 

 

 

 

 

Based upon its analysis of the Wisconsin lane data, EPA has concluded that although 

readiness seems to be a concern among the earliest OBD model years, flexibility in the form of 

readiness-based exemptions can go a long way toward minimizing the problem in the short term, 

while improvements in OBD implementation by the manufacturers will likely eliminate or 

greatly minimize the problem in the long run. To provide the needed flexibility to states to 

ensure smooth implementation of their OBD-based I/M programs, EPA is taking action to allow 

states to complete the testing process on MY 1996-2000 vehicles with two or fewer unset 

readiness codes; for MY 2001 and newer vehicles, the testing process can still be considered 

complete provided there is no more than one unset readiness code. This does not mean that these 

vehicles are exempt from the OBD-I/M check. The complete MIL check and scan must be run in 

all cases, and the vehicle still must be failed if the MIL is commanded on. The vehicle should 

continue to be rejected if it is MY 1996-2000 and has three or more unset readiness codes or is 

MY 2001 or newer and has two or more unset readiness codes. This allowance is consistent with 

a FACA OBD workgroup recommendation. It is intended to reduce the potential for customer 

inconvenience during this start-up phase of the transition to OBD-I/M testing. We believe that 

the environmental impact of this exemption will be negligible, given the small number of 

vehicles involved, the likelihood that at least some of these readiness codes will have been set in 

time for subsequent OBD-I/M checks, and the fact that an unset readiness code is not itself an 

indication of an emission problem. 

4.4 Gas Cap Testing vs. OBD-I/M 

4.4.1 Background 

Unlike the OBD exhaust test versus the IM240, where the failure criteria for OBD are 

tighter than the failure criteria for the IM240, the OBD failure criteria for leak detection are 

known to be more lenient than the gas cap pressure test currently in use in several states. 

Although in theory this difference in test stringency should result in a greater number of failures 

for the gas cap test than for the OBD-based evaporative system test, it is not obvious that 

vehicles actually develop such “in between” leaks in the real world (and, if so, whether the 

frequency of such leaks is significant enough to warrant recommending the continuation of the 

gas cap test in conjunction with OBD-based testing). 

To shed light on this issue, EPA decided to look at the Wisconsin data, focusing on the 

relative failure rates for the OBD-based evaporative system test versus the gas cap pressure test. 

Unlike the previous discussion concerning the use of gross failure rates as an indicator of test 

effectiveness when it comes to analyzing tailpipe tests, comparative failure rate is a fairly 

reliable index of a leak detection test’s relative effectiveness -- even when the failure criteria of 

the two tests being compared are different. Compared to tailpipe tests which are dynamic and 

relatively complicated, leak detection tests aimed at identifying leaks in the gas cap are 

straightforward. Whereas vehicle operation (i.e., whether steady-state, transient, loaded or 

unloaded), vehicle preconditioning, fuel composition, tire inflation, and multiple-instrument test 

equipment calibration can all have a considerable impact on the pass-fail decisions made by a 

traditional tailpipe test, none of these factors will have much effect on the traditional gas cap 

pressure test. 

38
 
 

 

 

 

 

 

4.4.2 Results 

Table 19 below compares the relative number and percentage of OBD evaporative system 

failure rates found in the Wisconsin vehicle sample versus the gas cap pressure test failure rate. 

Note that the gas cap failure rate is several orders of magnitude higher than the OBD failure rate 

for the entire evaporative emission system. Clearly, some of this is due to the fact that enhanced 

OBD evaporative system monitoring was phased in over the model years being looked at in this 

sample3 3 . Furthermore, as described in our earlier discussion on OBD readiness, the overall 

OBD readiness on MY 1998 LDVs was dominated by vehicles which showed up at the test lane 

without their evaporative system monitors having run. Even with these caveats taken into 

consideration, however, EPA believes that the difference in failure rates is pronounced enough to 

warrant consideration of retaining the gas cap pressure test as a complement to OBD-I/M testing 

in those areas needing VOC reductions to attain and/or maintain their clean air goals. 

Table 19: Gas Cap vs. OBD Evaporative System Failure Rates 

MY/Class Total Tested 

Gas Cap Fail 

(number) 

Gas Cap Fail 

(percent) 

OBD Evap Fail 

(number) 

OBD Evap Fail 

(percent) 

1996 LDV 27,313 291 1.1% 7 0.03% 

1996 LDT 16,422 245 1.5% 3 0.02% 

1996 Total 43,735 536 1.2% 10 0.02% 

1997 LDV 14,944 83 0.6% 2 0.01% 

1997 LDT 7,656 48 0.6% 1 0.01% 

1997 Total 22,600 131 0.6% 3 0.01% 

1998 LDV 27,616 170 0.6% 6 0.02% 

1998 LDT 22,716 155 0.7% 1 0.004% 

1998 Total 50,332 325 0.6% 7 0.01% 

4.4.3 Conclusions 

For the reasons discussed above, EPA believes that separate pressure testing of the gas 

cap test using the test procedures currently employed in many I/M programs should be continued 

in conjunction with OBD-I/M testing on MY 1996 and newer OBD-equipped vehicles. 

Retention of the gas cap pressure test is the only exception to EPA’s standing recommendation 

regarding the dual testing of MY 1996 and newer, OBD-equipped vehicles. 

33 The phase-in requirements for MY 1996, 1997, 1998, and 1999+ are 20%, 40%, 90% and 100%, respectively. 

39
 
 

 

 

 

 

 

 

 

 

 

 

 

 

Appendix 1: Test Sequence Used at Laboratories 

1) Procurement and acceptance into the program 

2) LA-4 cycle (preconditioning for IM240 test) 

3) IM240 test 

4) Drain in-use fuel 

5) Fill with indolene (40% fill) 

6) LA-4 cycle (preconditioning for FTP test) 

7) 12 hour soak (no diurnal heat build) 

8) FTP test (no evaporative test) 

9) IM240 test 

10) Repair if necessary 

11) OBD Readiness codes cleared thru operation of vehicle 

12) Repeat starting at step 4 
Appendix 2: Breakdown of the broken parts found for FTP-passing, MIL-on vehicles 

Systems/Components LDV LDT 

O2 Sensor 3 24 

EGR Valve 4 6 

Ignition System (spark plugs, ignition wires, other) 9 0 

Transmission components 3 5 

PCM, Reprogram or Replace 4 3 

Miscellaneous Wires 3 0 

Engine, Mechanical (cylinder head, harmonic balancer, valve springs) 2 1 

Vacuum Leaks 1 1 

Thermostat, Cooling System 1 0 

Fuel Pump 0 1 

Transmission Unit 0 1 

Cam Sensor 1 0 

Secondary Air Combo Valve 1 0 

Throttle Position Valve 0 1 

Exhaust Leak 0 1 

Catalyst 0 1 
 

 

Appendix 3: Lane IM240 vs. Lab IM240 

There are a number of differences between the way an IM240 test is conducted in an 

inspection lane and the way that the test is conducted in an emissions laboratory. Some of them 

are: 

1) quality of the test equipment 

2) frequency of calibration of test equipment 

3) skill of technician 

4) control of ambient conditions 

5) control of tire pressure 

6) operating temperature of the vehicle 

The first five items are of critical importance for a certification test in the laboratory but it 

is our opinion that they not crucial for the I/M function. By far the greatest importance is item 

six. There is a large variation in emissions between a partly warmed vehicle and a fully warmed 

vehicle. In the laboratory an LA4 ( 1372 seconds )test cycle is run before the LAB240 test to 

assure that the engine is fully warmed up and the catalyst hot. Vehicles arriving at I/M inspection 

lanes are assumed to be at operating temperature due to the driving prior to arrival at the lane 

(this may or may not be true). Attempts have been made in I/M systems to address this 

preconditioning problem through various methods. 
 

 

Appendix 4: Description of Vehicles/Trucks Assumed to Fail FTP 

CDH4, 1996 S-10 

Pickup MIL off 

(computer 

commanding MIL 

“On”) 

Truck could not accelerate and would 

stall in 3rd gear on FTP 

Lab IM240 results: 

(THC/CO/NOx) 

11.8/147/0.02 

Black plume of smoke from 

tailpipe 

ATL78, 1999 

Malibu 

MIL illuminated 

74,000 miles 

IM240 test of the vehicle caused 

closure of test cell due to hydrocarbon 

contamination of instruments. 

Decision made to not run FTP. 

Lab IM240 results: 

32.1/45.6/0.14 

Raw fuel out of the tailpipe 
Appendix 5 

Raw Data – OBD/Tailpipe Pilot 
Appendix 6 

Detailed Data From The 30-Vehicle OBD Evaporative Pilot Study 
 

 

 

Appendix 7 

This section will primarily discuss issues with the Mazdas (150, 182), Hondas (153, 188), and 

Fords (155, 194). Reports and data are still pending from Honda and Ford, respectively. 

Use of external mounted thermocouples instead of installing internal thermocouples is a 

common EPA practice for in-use evaporative emission testing. Without this simplification, 

instrumenting the vehicle in strict accordance with the EPA certification requirements for 

locating thermocouples and fuel drains can require cutting access panels through the floor of the 

vehicle’s trunk compartment. ATL’s practical experience in using surface mounted 

thermocouples is that this thermocouple location does not compromise testing accuracy because 

accurate measurement of the internal liquid temperature at the mid-point of the 40% fuel level is 

achieved. Vehicles with plastic fuel tanks used thermocouple probes installed through the 

bottom of the fuel tank. Any fuel tank modification that comprised the integrity of the OEM 

tank was resolved by installing a new fuel tank or fuel sending unit before the vehicle was 

returned to the owner. 

The FTP evaporative emission running loss test requires that the measured fuel tank 

temperature track the target temperature within 3 degrees F over the dynamometer driving 

portion (Urban Dynamometer Driving Schedule (UDDS), New York City Cycle (NYCC), 

NYCC, UDDS) of the running loss test. In general, the measured fuel tank temperatures denoted 

as “Actual F” (failed) or “Actual R” (repaired) in Table A-5 in the Appendix indicate close 

agreement with the vehicle manufacturer supplied fuel tank temperature profile. Manufacturer 

supplied fuel tank target temperatures and ATL measured temperatures for starting and ending 

segments of the running loss test are summarized in Table A-5 in the Appendix. 

Exceptions to meeting the 3 degree tolerance were observed for vehicles 150, 154, 184, 

and 189. The 3 degree tolerance is not straightforward to meet for in-use evaporative emission 

testing. The deviations for vehicles 150, 154, 184, and 189 range from slightly over 3 degrees F 

to about 7 degrees F. Not withstanding test work with vehicle 188, a 1999 Honda Accord 

(discussed below), these deviations from the target temperature profile and the short time of the 

excursion are not thought to be important because their effect on running loss results is judged to 

be minor. 
 

 

 

 

 

 

 

 

 

 

Appendix 8: Wisconsin I/M and OBD Data Fields 

Date/time Date and time the vehicle was tested 

Mod yr. Model year 

Make Make 

Model Model 

Vin Vin number 

Test Test number, 1 for the first time vehicle has been tested in this test cycle, 2 for the first retest. A very few vehicles 

have been retested four times. 

HC Stan Final cutpoints, 0.6 grams per mile for cars 

Co Stan Final cutpoints, 1.5 grams per mile for cars 

NOx Final cutpoints 0.7 grams per mile for cars, Wisconsin does not fail for NOx 

HC Actual emissions total grams divided by the total miles, at the time the test was terminated 

Co Actual emissions total grams divided by the total miles, at the time the test was terminated 

NOx Actual emissions total grams divided by the total miles, at the time the test was terminated 

Em fsec Number of seconds that the test ran. ( “0” for the full 240 second test) 

Em res P or F, pass or fail the 240 tailpipe test 

Pr cap Stan Pressure cap standard, inches of water 

Pr cap ini Initial pressure, inches of water 

Pr cap Final pressure, inches of water 

Pr cap res P or F, pass or fail pressure cap test 

Onboard Whether the technician could find the OBD connection. No cases where he could not after October 98 

Obd res Pass or fail, if MIL was illuminated. Should correspond to column AM 

Tr no Number of codes present (sum of V through AA) but is sometimes wrong 

Code1 The next six columns list the OBD trouble codes, if any 

Code2 - 6 (Blank) 

Ready misfire The next 11 columns list the readiness codes. 0 means that the monitor is not fitted. 1 means that the monitor is fitted 

but not ready. 2 means that the monitor is ready 

Fuel Fuel trim 

Comp Various circuits necessary for the other monitors to work 

Cat Catalyst 

Hcat Heated catalyst 
 

Evap Evaporative system 

Sair Secondary air 

Acsys Air conditioning 

Oxy Oxygen sensor 

Hoxy Heated oxygen sensor 

Egr Exhaust gas recirculation sensor 

Obd Mil Mil light, 1 if lighted, 0 if not. Should be same as column T 

Odo Odometer reading to nearest 1,000 miles (truncated)