Development and Evaluation of 

Colorado's Low Emitter Index 

Colorado Department of Public Health and Environment 

May 2007 

Prepared by: 

Mobile Sources 

Air Pollution Control Division 

Colorado Department of Public Health and Environment 

and 

Peter M McClintock, PH.D. 

Applied Analysis 

891 Tiburon Blvd. 

Peter.mcclintock@comcast.net 
•Page 2 
Introduction 

Colorado's inspection and maintenance (l/M) program includes a Clean Screen element that allows 

vehicle owners the option of being exempted from their required l/M240 inspection, if their vehicle is 

determined clean. Currently, to qualify as clean a vehicle must be seen by a Remote Sensing Device 

(RSD) a minimum of two times within 12 months prior to its required l/M240 inspection, with emissions 

measurement less than 200ppm and .5% for HC and CO respectively. 

The Colorado Department of Public Health and Environment (CDPHE) is proposing to add a hybrid 

method of identifying clean vehicles that combines a single clean RSD measurement with a Low 

Emitter Index (LEI) criterion. The LEI qualifies vehicles as Clean Screen candidates based on a 

historically low probability of emissions failure for similar vehicles. The addition of the LEI hybrid 

method will increase the productivity of the existing Clean Screen program and allow more vehicle 

owners to take advantage of the convenience of clean screening without jeopardizing the benefit to the 

current l/M program. The LEI qualification criteria will be evaluated/updated on an annual basis to 

ensure it is equal to or better than the current 2-hit clean screen program. 

Colorado's l/M program is included in both the One Hour Ozone Maintenance Plan and Eight Hour 

Ozone Early Action Compact (EAC), as a mobile source control strategy. The program as modeled 

prohibits Colorado from evaluating more than 50% of the l/M eligible fleet. The inclusion of the 

proposed LEI hybrid method will not exceed this threshold. 

This document describes how the LEI was developed along with the comparison of the projected 

performance of the hybrid method to the current 2-hit Clean Screen method. 

Source Data and Vehicle Grouping 

An LEI lookup table hasb een developed using 2005 and 2006 initial IM240 inspection results. 

Colorado tests AWD and traction control vehicles on 4WD dynes so there are few models that do not 

receive IM240. 

Vehicles are grouped using: 

Colorado 1 Vehicle Type: LDGV, LDGT1 , LDGT2, HDGT1 , HDGT2 

Model Year: the tenth character from the VIN 

Make: decoded from the VIN; 

Model: decoded from the VIN; 

Engine Displacement: decoded from the VIN; 

Only groups containing 20 or more vehicles will be retained in the combined profile in order to avoid 

concerns over statistical variability in small sample sizes. Missouri had used the first five characters of 

the VIN in lieu of Make and Model. Colorado proposes using Make and Model as decoded from the 

VIN using the Polk program, since this approach appears to better and more consistently differentiate 

Make/Model than the Missouri approach. 

1 In the Colorado data, light trucks are defined using four types: LDGT1, LDGT2, HDGT1, and HDGT2. 

The first pair roughly corresponds to the EPA's LDGT1 and the second pair corresponds to EPA's 

LDGT2. 

•Page 3 
Table 1 compares the use of alternative sets of LEI vehicle groupings. It shows the number of source 

vehicles, the number of bins with 20 or more results, and the percentage of all source vehicles in these 

classifications. Coverage of source vehicles is highest with the proposed scheme. 

Table 1 

LEI Vehicle Vehicles >=20 HC/CO NOx HC/CO + Fleet 

Groupings Bins Fail% Fail% NOxFail % Coverage 

% 

None (All vehicles) 1,557,509 N/A 2.67% 0.16% 2.83% 100% 

Veh. Type, Model Year , First 5 

VIN Char., Polk Engine 1,439,382 8,438 2.54% 0.16% 2.7% 92.4% 

Displacement, # of Cvlinders 

Veh. Type, Model Year , First 5 

VIN Char., Polk Engine 1,450,518 8,388 2.53% 0.16% 2.69% 93.1% 

Displacement 

Veh. Type, Model Year, Polk 

Make, Polk Model, Polk Engine 1,476,842 6,817 2.58% 0.16% 2.74% 94.8% 

Disolacement 

The LEI table generated using 2005/2006 l/M test results will typically be applied to vehicles inspected 

in 2007 and part of 2008. Colorado has a four-year exemption for new vehicles. Although some 

inspections on these models are performed, the 2005/2006 inspection results may leave gaps in the 

profile for models that will be four-year old vehicles in 2007/2008, e.g. 2003 and 2004 models. The 

gaps will be filled to a large extent by assuming newer year vehicles, will perform similarly (or better) 

than the same model of an earlier year, for which performance statistics are available, e.g. 2003 and 

newer Acura MDXs will perform at least as well as 2002 Acura MDXs. 

LEI Emissions Perfonnance 

The values listed below were computed for each vehicle group. The average excess emissions per 

vehicle are the sum of the vehicle emissions exceeding the Colorado standards divided by the number 

of vehicles in the group. These g/mi values are usually small compared to g/mi standards because 

most vehicles do not add any excess emissions and all vehicles are included in the denominator. 

Initial tests 

HC/CO fails 

HC/COfail % 

NOxfails 

NOxFail% 

Overall fail% 

Average Excess HC g/mi per vehicle 

Average Excess CO g/mi per vehicle 

Average Excess NOx g/mi per vehicle 

The LEI clean screening effectiveness will be targeted towards the most critical pollutants by adjusting 

the selection criteria. · 

Figure 1A plots the cumulative percentage of excess emissions vs. the percentage of vehicles when 

LEI groups are ranked by tailpipe emissions fail rate. Figure 1 B is the same chart with an expanded yaxis. These charts indicate when groups of vehicles are ranked by increasing tailpipe fail rate, groups 

containing 70% of the vehicles produce only 10% of the excess HC and CO and 30% of the excess 

NOx. Note that total excess NOx is small compared to excess HC and CO due to Colorado's lenient 

NOx standards. 

• Page4 
Figure 1A LEI Excess Emissions vs.% of Vehicles When Ranked by Tailpipe Fail Rate 

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--Excess HC% 

-Excess CO% 

0 10 20 30 40 50 60 70 80 90 

% of Vehicles 

Figure 1 B LEI Excess Emissions vs. % of Vehicles When Ranked by Tailpipe Fail Rate 

Indexed by Tailpipe Fail Rate 

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0 10 20 30 40 50 60 70 80 90 

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•Page 5 
Figures 2A and 28 show the cumulative percentage of excess emissions vs. the percentage of vehicles 

when LEI groups are ranked by the fail rate for HC or CO. Seventy percent of the vehicles produce 8% 

of the excess HC and CO. 

Figure 2A LEI Excess Emissions vs. % of Vehicles When Ranked by HC/CO Fail Rate 

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--Excess HC% 

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0 10 20 30 40 50 60 70 80 90 

% of Vehicles 

Figure 28 LEI Excess Emissions vs.% of Vehicles When Ranked by HC/CO Fail Rate 

Indexed by HC/CO Fail Rate 

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• Page6 
Figures 3A and 38 show the cumulative percentage of excess emissions vs. the percentage of vehicles 

when LEI groups are ranked by excess HC. Seventy percent of the vehicles produce 5% of the excess 

HC. 

•Page 7 

Figure 3A LEI Excess Emissions vs. % of Vehicles When Ranked by Excess HC 

Indexed by Excess HC 

--Excess HC% I/) 

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Figure 38 LEI Excess Emissions vs. % of Vehicles When Ranked by Excess HC 

~ 25% 

.2 

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Indexed by Excess HC 

--Excess HC% • 

-Excess CO% 

- - -•- - -Excess NOxOf<M' "" ~ 20% -1-~~~~~~...,......~f----+-~-Hl---J--+-<----f 

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0 10 20 30 40 50 60 70 80 90 

% of Vehicles 
LEI Clean Screen Selection Criteria 

To maintain flexibility in the selection of LEI vehicles (both quantity and quality), it is suggested that the 

following five criteria be considered. Colorado is proposing using only the HC/CO Fail Rate criteria 

however the remaining four will continue to be calculated and if necessary included as part of future LEI 

tables. 

Overall tailpipe fail rate is less than: 

HC/CO Fail Rate is less than 

Excess HC is less than: 

Excess CO is less than: 

Excess NOx is less than 

Xo/o 

Yo/o 

Ag/mi 

Bg/mi 

Cg/mi 

To ignore one or more of the criteria, either a high or a low value is entered. For example, to select all 

vehicle classes with an overall fail rate of less than 1 %, entries would be 1 %, 99%, 99, 99 and 99. 

Average excess emissions per vehicle relate more directly to mass emissions benefits than fail rate. 

Given two groups of vehicles with the same fail rate, one group containing poor fuel economy vehicles 

and one group containing good fuel economy vehicles, the group with the poor fuel economy vehicles 

will contribute greater excess emissions. However, using fail rate as the criteria is easier for the media 

and the public to understand. 

Hybrid Method Evaluation 

The hybrid method of selecting vehicles for clean screen was evaluated using 2006 data. l/M results of 

vehicles that would have been selected using the hybrid method are compared to the results of the 5% 

audit sample of vehicles passing the current 2-hit screen and vehicles that were identified as clean (2-

hits) but for unknown reasons did not participate in the current clean screen program. 

Table 2 below shows that projected results for vehicles selected using the hybrid method compare 

favorably to the current RSD 2-hit method. Both the tailpipe and overall fail · rates, which includes 

evaporative test failures, are 0.3% lower. The hybrid method was modeled using RSD CO and HC 

cutpoints of 0.5% and 200ppm respectively and a simple LEI HC/CO fail rate of 2%. 

Table 2 Clean Screen Method Evaluation Summary 

Number IM240 

Method Sample of Tailpipe Overall l/M 

Fail Fail Rate Vehicles Rate 

Hybrid Sample 31,644 0.34% 2.6% 

2006 RSD 2-hit 5% Audit Sample 1,509 0.46% 2.5% 

Other RSD 2-hit Sample Meeting 15,299 0.66% 2.9% Screen Criteria 

Combined RSD 2-hit Samples 16,808 0.64% 2.9% 

•Page 8 
The results show that the effectiveness of the hybrid method is equivalent to that of the current clean 

screening method using two RSD measurements. 

Further investigation is being conducted to determine if the evaporative failures can be cost effectively 

reduced for both methods by applying an additional restriction on clean screens for models with a 

pattern of evaporative test failures. 

The remainder of this section provides details of the hybrid evaluation. 

Hybrid Evaluation Process 

As noted above, the hybrid method of selecting vehicles for clean screen was evaluated using 2006 l/M 

results of vehicles that would have been selected using this method. 

Vehicle owners are notified of their clean screen eligibility one to two months prior to their registration 

renewal date. Change of ownership vehicles that are inspected off-cycle are, therefore, not able to take 

advantage of clean screening. Historically, vehicles changing ownership have higher emissions and 

failure rates than other vehicles. To accurately project hybrid clean screen program performance these 

off-cycle tests are removed from the evaluation sample. To accomplish this, vehicles tested in 2006 

were selected where their first test occurred within 23-25 months of their first test in 2004. This 

identified 303,525 vehicles tested with IM240. 

RSD measurements from the 12-month window in advance of the IM240 test were identified for the 

303,525 vehicles. Of these, 23,521 vehicles had at least two measurements and 48,438 vehicles had 

a single measurement. 

Vehicles with two measurements would typically be considered for current 2-hit screening method. 

Vehicles that accepted the clean screen option would not typically have an IM240 test in 2006 and, 

therefore, are not typically included in the extracted sample of vehicles with two measurements 

described above. The extracted sample of vehicles with two measurements does include the 5% audit 

sample, vehicles that did not pass the screen, vehicles that did not accept the clean screen option and 

vehicles for which the second qualifying RSD may not have been in time to be included before the 

processing deadline. 

Vehicles with a single measurement passed the hybrid method if their HC and CO emissions were 

below 200ppm and 0.5% respectively on their RSD measurement and their LEI group had an HC/CO 

fail rate of less than 2%. Results are compared to both the 5% audit sample and the other RSD 2-hit 

vehicles screened using the current 2-hit method. 

Since vehicles with two qualifying measurements would not be eligible for the hybrid method they were 

not considered part of the hybrid evaluation. However, vehicles from this group that would have met 

the 2-hit RSD screen criteria provide another point of reference for comparison with the effectiveness of 

the hybrid method. Therefore, results for three samples are considered in the following charts and 

tables: 

• 2006 RSD 2-hit Audit sample 

• Hybrid Method sample 

• Other RSD 2-hit sample; vehicles with two RSD measurements that met the 2-hit screen 

criteria. 

Figure 4 shows that MY distributions of vehicles meeting the screening criteria from the 'Hybrid Sample' 

and the 'Other 2-Hit Sample' vehicles are similar to the 'Audit Sample' except that the 'Audit Sample' 

includes more 2002 models. Most 2002 models were not tested in 2004 and, as a result of the sample 

•Page 9 
selection method described above, were not included in the 'Hybrid Sample' or the 'Other 2-hit 

Sample'. Since the 2002 models are the newest full model year, and are expected to be virtually all 

clean, their omission does not compromise the hybrid evaluation although the evaluated hybrid 

effectiveness may appear to be less good than if all these 2002 models had been included. 

Figure 4 - Vehicle Age Distribution of Clean Screen Vehicles 

Age Distribution of Vehicle Samples Meeting Clean 

Screen Criteria 

--Hybrid 

25% --RSD2-hit 1---~~~~~~~~~~~~~~~~--j 

- s% Audit 

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Figure 5 shows the projected hybrid pass rate vs. the current Clean Screen program by model year in 

2006. The pass rate is higher for the newest models but drops rapidly below the current program for 

1996 and older models, which were over ten years old in 2006. The overall hybrid pass rate was 68% 

of candidates vs. 78% for current 2-hit RSD. 

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•Page 10 

Figure 5 - Hybrid vs. Current Clean Screen Program Pass Rate 

100 % 

90 % 

80 % 

70 % 

60 % 

50 % 

40 % 

30 % 

20 % 

10 % 

0 % 

Pass Rate of Clean Screen Candidatesi n 2006 and 

Projected Hybrid Method 

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Tables 3, 4 , 5 and 6 show the IM240 test results for the 'Audit Sample', 'Hybrid Sample', 'Other RSD 

2-hit Sample', and 'Audit Sample and Other RSD 2-HitS ample Combined'. The tables show the MY 

distribution, the tailpipe fail rate, the overall fail rate and the average IM240 g/mi emissions of vehicles 

passing the screens. 

Despite the absence of a full complement of 2002 models, the average emissions of vehicles passing 

the Hybrid method are lower than the 'Audit Sample' by 0.01 , 0.1 and 0.01 g/mi for HC, CO and NOx 

respectively. 

The average emissions of vehicles passing the Hybrid method are lower than the 'Other RSD 2-hit 

Sample' by 0.05, 0.7 and 0.09 g/mi for HC, CO and NOx respectively. 

The Hybrid method is therefore at least as effective as the RSD 2-hit method. 

Table 3 -Audit Sample 

2006 Audit Sample Vehicles Passing 2-Hit Screen with RSD CO <0.5% and HC <200ppm 

Model Fraction IM240 IM Model Vehicles IM240 Overall IM240 IM240 IM240 

Year of Tailpipe Overall Year Tailpipe Failures Avg. Avg. Avg. 

Screen Fail Fail Failures HC co NOx 

Passes Rate Rate g/mi g/mi g/mi 

1982 0% 0.00% 0.0% 1982 0 0 0 - - - 1983 0% 0.00% 0.0% 1983 0 0 0 - - -

1984 0% 0.00% 0.0% 1984 1 0 0 4.37 21 .2 4.20 

1985 0% 0.00% 0.0% 1985 1 0 0 0.98 12.9 2.02 

1986 0% 0.00% 0.0% 1986 2 0 0 1.92 14.7 2.87 

1987 0% 0.00% 0.0% 1987 2 0 0 1.12 8.1 1.72 

1988 1% 7.69% 7.7% 1988 13 1 1 1.45 10.7 1.82 

1989 1% 0.00% 9.1% 1989 11 0 1 0.93 7.8 2.19 

1990 1% 4.76% 9.5% 1990 21 1 2 1.04 12.2 2.00 

1991 1% 0.00% 0.0% 1991 15 0 0 0.78 8.8 1.69 

1992 2% 0.00% 0.0% 1992 23 0 0 1.04 9.7 2.07 

1993 2% 0.00% 2.9% 1993 34 0 1 0.74 7.3 1.47 

1994 3% 0.00% 0.0% 1994 50 0 0 0.56 5.8 1.35 

1995 4% 1.54% 7.7% 1995 65 1 5 0.62 5.5 1.30 

1996 6% 0.00% 2.1% 1996 96 0 2 0.38 4.3 1.28 

1997 6% 0.00% 1.0% 1997 98 0 1 0.37 4.1 1.30 

1998 12% 0.56% 2.2% 1998 178 1 4 0.29 3.6 1.03 

1999 8% 0.84% 2.5% 1999 119 1 3 0.30 3.0 1.02 

2000 17% 0.78% 1.9% 2000 258 2 5 0.20 2.8 0.68 

2001 10% 0.00% 2.6% 2001 153 0 4 0.12 2.2 0.48 

2002 22% 0.00% 2.4% 2002 338 0 8 0.11 1.8 0.47 

2003 2% 0.00% 0.0% 2003 31 0 0 0.14 2.6 0.60 

2004 0% 0.00% 0.0% 2004 0 0 0 - - -

2005 0% 0.00% 0.0% 2005 0 0 0 - - -

Total 100% 0.46% 2.5% 1,509 7 37 0.31 3.6 0.90 I 

•Page 11 
Table 4 - Hybrid Sample 

Projected 2006 Hybrid Screen Passes: RSD CO <0.5% & HC < 200ppm, LEI HC/CO Failure < 2% 

Model Fraction IM240 IM Model Vehicles IM240 Overall IM240 IM240 IM240 

Year of Tailpipe Overa'll Year Tailpipe Failures Avg. Avg. Avg. 

Screen Fail Fail Failures HC co NOx 

Passes Rate Rate g/mi g/mi g/mi 

1982 0% 0.00% 0.0% 1982 1 0 0 4.00 31 .8 3.10 

1983 0% 0.00% 0.0% 1983 0 -0 0 - - -

1984 0% 6.25% 18.8% 1984 16 1 3 1.76 20.7 2.14 

1985 0% 0.00% 16.7% 1985 6 0 1 1.43 14.8 1.93 

1986 0% 0.00% 0.0% 1986 38 0 0 0.99 11 .1 1.92 

1987 0% 0.00% 18.2% 1987 11 0 2 0.99 10.2 2.17 

1988 0% 0.00% 3.3% 1988 30 0 1 0.95 9.4 1.53 

1989 0% 0.00% 3.1% 1989 32 0 1 0.79 8.4 1.68 

1990 1% 0.56% 3.3% 1990 180 1 6 0.82 7.8 1.76 

1991 1% 0.00% 4.2% 1991 236 0 10 0.76 7.7 1.74 

1992 2% 0.16% 2.3% 1992 607 1 14 0.67 7.1 1.53 

1993 2% 1.15% 4.3% 1993 697 8 30 0.70 7.3 1.71 

1994 3% 0.91% 3.4% 1994 993 9 34 0.56 5.9 1.34 

1995 5% 0.84% 3.1% 1995 1,426 12 44 0.51 5.1 1.34 

1996 9% 0.46% 3.5% 1996 2,820 13 98 0.38 4.2 1.13 

1997 9% 0.51% 3.8% 1997 2,943 15 113 0.36 4.1 1.11 

1998 16% 0.29% 2.0% 1998 5,214 15 103 0.30 3.4 0.91 

1999 12% 0.21% 2.3% 1999 3,781 8 87 0.27 3.0 0.86 

2000 24% 0.23% 2.3% 2000 7,538 17 170 0.20 2.5 0.66 

2001 8% 0.17% 2.2% 2001 2,387 4 52 0.14 2.1 0.48 

2002 5% 0.25% 2.3% 2002 1,601 4 37 0.12 2.0 0.44 

2003 2% 0.00% 1.2% 2003 753 0 · 9 0.09 1.7 0.36 

2004 1% 0.00% 1.2% 2004 326 0 4 0.08 1.5 0.29 

2005 0% 0.00% 0.0% 2005 8 0 0 0.07 1.3 0.28 

Total 100% 0.34% 2.6% 31,644 108 819 0.30 3.5 0.89 

•Page 12 
Table 5 - Other RSD 2-Hit Sample 

Other Vehicles Passing 2-hit Screen with CO <0.5% and HC <200ppm 

Model Fraction IM240 IM Model Vehides IM240 Overall IM240 IM240 IM240 

Year of Tailpipe Overall Year Tailpipe Failures Avg. Avg. Avg. 

Screen Fail Fail Failures HC co NOx 

Passes Rate Rate g/mi g/mi g/mi 

1982 0% 0.00% 0.0% 1982 4 0 0 1.96 15.6 3.15 

1983 0% 0.00% 0.0% 1983 2 0 0 1.58 11.5 4.15 

1984 0% 0.00% 7.1% 1984 14 0 1 1.88 21 .2 2.46 

1985 0% 4.35% 4.3% 1985 23 1 1 1.15 14.1 2.33 

1986 0% 2.22% 4.4% 1986 45 1 2 1.30 13.9 2.28 

1987 0% 5.45% 9.1% 1987 55 3 5 1.13 11 .9 2.33 

1988 1% 1.71% 4.3% 1988 117 2 5 1.13 11.9 1.95 

1989 1% 2.70% 5.4% 1989 111 3 6 1.04 11.6 1.94 

1990 2% 0.87% 6.1% 1990 230 2 14 0.89 8.7 1.79 

1991 2% 1.96% 7.5% 1991 255 5 19 0.84 8.8 1.83 

1992 3% 3.42% 7.8% 1992 409 14 32 0.82 9.4 1.72 

1993 3% 1.61% 4.6% 1993 436 7 20 0.73 7.9 1.86 

1994 4% 1.53% 4.1% 1994 589 9 24 0.70 7.4 1.59 

1995 5% 2.08% 5.6% 1995 818 17 46 0.63 6.2 1.45 

1996 8% 0.51% 2.1% 1996 1,167 6 25 0.41 4.7 1.26 

1997 8% 0.89% 3.7% 1997 1,237 11 46 0.40 4.4 1.17 

1998 13% 0.63% 2.3% 1998 2,054 13 48 0.31 3.9 0.97 

1999 10% 0.06% 2.0% 1999 1,578 1 32 0.27 3.2 0.88 

2000 20% 0.20% 2.2% 2000 3,037 6 67 0.20 2.7 0.66 

2001 7% 0.00% 2.3% 2001 1,142 0 26 0.13 2.2 0.50 

2002 5% 0.00% 1.6% 2002 770 0 12 0.11 1.8 0.43 

2003 5% 0.00% 1.3% 2003 775 0 10 0.09 1.6 0.36 

2004 3% 0.00% 0.5% 2004 420 0 2 0.08 1.5 0.29 

2005 0% 0.00% 0.0% 2005 11 0 0 0.07 1.6 0.19 

Total 100% 0.66% 2.9% 15,299 101 443 0.35 4.2 0.98 

•Page 13 
Table 6 - Audit Sample and Other RSD 2-Hit Sample Combined 

2006 Audit Sample+ Other 2-Hit Passing 2-Hit Screen with RSD CO <0.5% and HC <220ppm 

Model Fraction IM240 IM Model Vehicles IM240 Overall IM240 IM240 IM240 

Year of Tailpipe Overall Year Tailpipe Failures Avg. Avg. Avg. 

Screen Fail Fail Failures HC co NOx 

Passes Rate Rate g/mi g/mi g/mi 

1982 0% 1982 4 - - 1.96 15.61 3.15 

1983 0% 1983 2 - - 1.58 11 .53 4.15 

1984 0% 0.00% 6.7% 1984 15 - 1 2.05 21 .23 2.58 

1985 0% 4.17% 4.2% 1985 24 1 1 1.15 14.04 2.31 

1986 0% 2.13% 4.3% 1986 47 1 2 1.33 13.96 2.30 

1987 0% 5.26% 8.8% 1987 57 3 5 1.13 11 .77 2.31 

1988 1% 2.31% 4.6% 1988 130 3 6 1.17 11 .80 1.94 

1989 1% 2.46% 5.7% 1989 122 3 7 1.03 11 .27 1.96 

1990 1% 1.20% 6.4% 1990 251 . 3 16 0.91 9.00 1.81 

1991 1% 1.85% 7.0% 1991 270 5 19 0.84 8.83 1.82 

1992 3% 3.24% 7.4% 1992 432 14 32 0.84 9.43 1.74 

1993 3% 1.49% 4.5% 1993 470 7 21 0.73 7.89 1.83 

1994 4% 1.41% 3.8% 1994 639 9 24 0.69 7.28 1.57 

1995 5% 2.04% 5.8% 1995 883 18 51 0.63 6.19 1.44 

1996 8% 0.48% 2.1% 1996 1,263 6 27 0.41 4.71 1.27 

1997 8% 0.82% 3.5% 1997 1,335 11 47 0.39 4.40 1.18 

1998 13% 0.63% 2.3% 1998 2,232 14 52 0.31 3.86 0.97 

1999 10% 0.12% 2.1% 1999 1,697 2 35 0.27 3.16 0.89 

2000 20% 0.24% 2.2% 2000 3,295 8 72 0.20 2.69 0.67 

2001 8% 0.00% 2.3% 2001 1,295 - 30 0.13 2.15 0.50 

2002 7% 0.00% 1.8% 2002 1,108 - 20 0.11 1.82 0.44 

2003 5% 0.00% 1.2% 2003 806 - 10 0.09 1.67 0.37 

2004 2% 2004 420 - 2 0.08 1.51 0.29 

2005 0% 2005 11 - - 0.07 1.58 0.19 

Total 100% 0.64% 2.9% 16,808 108 480 0.35 4.1 0.97 

•Page 14 
United States 

Environmental Protection 

Agency 

Air and Radiation 

&EPA Performing Onboard 

Diagnostic System 

Checks as Part of a 

Vehicle Inspection and 

Maintenance Program 

EPA420-R-01-015 

June 2001 

@ Printed on Recycled Paper 
EPA420-R-01-015 

June 2001 

Performing Onboard Diagnostic System Checks as 

Part of a Vehicle Inspection and Maintenance Program 

David Sosnowski 

Edward Gardetto 

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 i~ 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. 
Performing Onboard Diagnostic System Checks as Part of a Vehicle Inspection and 

Maintenance Program 

June 2001 

Introduction 

The Clean Air Act as amended in 1990 (CAA) requires the Environmental Protection 

Agency (EPA) to set guidelines for states to follow in designing and running vehicle inspection 

and maintenance (I/M) programs. As well as distinguishing between basic and enhanced I/M 

programs, these guidelines must clarify how states are to meet other minimum design 

requirements set by the CAA. One such requirement that applies to both basic and enhanced I/M 

programs is the performance of Onboard Diagnostic (OBD) system checks as part of the 

required, periodic inspection. 

On November 5, 1992, EPA published the I/M rule to meet most of the above-referenced 

CAA requirements. At the time the I/M rule was promulgated, however, federal OBD 

certification standards had not been published. To address the CAA's OBD-1/M requirement, 

EPA reserved sections in the 1992 rule, with the understanding that these reserved sections 

would be amended at some future date. Although the federal requirement to incorporate OBD 

into new vehicles began with the 1994 model year (MY), manufacturers were allowed to request 

waivers on vehicles for MY 1994-95, so full compliance was not required on all light-duty cars 

and trucks sold in this country until MY 1996. On August 6, 1996, EPA published amendments 

to the 1992 I/M rule establishing OBD-1/M requirements for I/M performance standards and I/M 

State Implementation Plans (SIPs). 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 an OBD-1/M pass, failure, or rejection. Lastly, the August 6, 1996 

amendments revised 40 CFR part 85, subpart W to establish OBD-1/M as an official performance 

warranty short test under section 207(b) of the Act. 

At the time the original OBD-1/M requirements were established, it was not practical to 

evaluate the real-world, in-use performance of OBD because the vehicles in question were still 

too new and the number of those vehicles in need of repair were too few to make pilot testing 

worthwhile. Therefore, in 1998, EPA further amended its OBD-I/M requirements to delay the 

date by which I/M programs must begin OBD testing to no later than January 1, 2001. 

One of the primary reasons for delaying the deadline for beginning OBD-I/M testing was 

to give EPA time to evaluate the OBD check as an I/M program element and to give states time 

to prepare for implementation. In conducting its evaluation of OBD, however, EPA found that 

identifying and recruiting OED-equipped vehicles in need ofrepair proved more difficult and 

time-consuming than originally anticipated. As a result, EPA has only recently completed the 

assessment of OBD-1/M effectiveness and implementation issues referenced in this guidance. 

2 
During the course of this evaluation, however, it became clear that certain regulatory changes 

were needed to .ensure the smooth implementation of OBD-1/M testing by the states. 

In response to its findings on OBD effectiveness and its study of the various 

implementation issues associated with OBD-1/M testing, EPA has amended the OBD-1/M testing 

requirements by publishing a final rulemaking (FRM) in the Federal Register on April 5, 2001 1

• 

The goal of these amendments is to update and streamline requirements and to remove regulatory 

obstacles that may otherwise impede the effective implementation of OBD-1/M testing. Among 

other things, the revised requirements: 1) provide states several options for extending the current 

deadline for mandatory implementation of the OBD-1/M inspection beyond January 1, 2001; 2) 

clarify states' options regarding the integration of OBD-1/M checks into existing l/M networks; 

3) revise and simplify the current list of Diagnostic Trouble Codes (DTCs) that constitute the 

OBD-1/M failure criteria to include any DTC that leads to the dashboard Malfunction Indicator 

Light (MIL) being commanded on; and 4) provide for exemptions from specific readiness code 

rejection criteria on OED-equipped vehicles based upon vehicle model year. 

In addition to the above cited regulatory revisions, EPA believes it is important to 

respond to states' requests to provide additional guidance on how to successfully implement 

OBD-1/M testing in an l/M program. EPA is therefore issuing this guidance at this time in 

response to those requests and to assist those states and local areas that are considering or 

planning early implementation of OBD checks as part of their l/M programs. 

Scope of Guidance 

This guidance incorporates several key recommendations made to EPA by the OBD 

Workgroup, which is part of the Mobile Source Technical Review Committee, established under 

the Federal Advisory Committee Act (F ACA). This guidance was also developed by drawing 

from the experiences of several states that are currently performing some form of OBD-based 

inspection. As of this writing, ten states (New York, Califomia2, Colorado, Alaska, Illinois, 

Wisconsin, Vermont, Oregon, Maine, and Utah3

) are performing some form of vehicle OBD 

system check and at least three other states (Indiana, New Hampshire, and Georgia) are actively 

moving towards early implementation of vehicle OBD system checks. Lastly, the development 

of this guidance was aided by comments received from stakeholders in response to an earlier, 

draft guidance released for public comment in December 2000. 

1 Copies of the FRM are available via the Internet at www.epa.gov/oms/epg/regs.htm. 

2 Currently, California and New York are performing only a visual check for MIL illumination. A scanner check for 

trouble codes will be added in the future. 

3 In Utah, the l/M program is administered at the county level (as opposed to at the state level). Although there are 

several counties in Utah currently required to implement UM, only one county - Davis - has opted to begin early implementation 

of the OBD-1/M check. 

3 
This guidance reflects EPA's current understanding of the challenges and issues unique to 

the performance of OED testing in the VM program environment, and includes our 

recommendations for how best to address those issues at this time. As is the case with any 

technology-driven pollution control measure, our understanding of the issues and the issues 

themselves are likely to change over time as we gain more experience with them. Therefore, 

EPA will update this guidance from time to time, as developments warrant. To ensure that this 

document is updated accurately, EPA invites those involved with performing the OED-VM check 

to share information with us by contacting Ed Gardetto at 734-214-4322, or via e-mail at 

gardetto.edward@epa.gov. Program data, recommendations, and other forms of feedback 

relevant to the performance of the OED-VM check can also be sent to: 

Ed Gardetto c/o 

U.S. Environmental Protection Agency 

2000 Traverwood Drive 

Ann Arbor, MI 48105 

Lastly, this guidance does not address those VM implementation issues which are 

common to all test types and for which there are no unique, OED-specific considerations. For 

example, this guidance does not address geographic coverage requirements or the adequacy of 

program funding mechanisms. Readers can find EPA's requirements and/or recommendations 

for these generic VM implementation issues by consulting the VM rule (as amended) and EPA's 

subsequent VM policy documents, which are available via the Office of Transportation and Air 

Quality (OTAQ) web site at: http:\\www.epa.gov\otaq\im.htm. 

Vehicle OBD System Checks: Basic Requirements 

Recommended Model Year Coverage 

Although some variety of OED system has been an option on certain vehicle models 

since the early 1980's, standardized OED systems (also known as OED II) were not introduced 

until MY 1994, and such systems did not appear on all new light-duty vehicles sold in this 

country until MY 1996. Therefore, for VM purposes, EPA does not require that pre-1996 MY 

vehicles be subject to the OED inspection discussed in this guidance. Furthermore, EPA does 

not recommend that such testing include MY 1994-95 vehicles because not all such vehicles are 

OED-equipped and the availability to manufacturers of limited waivers from some OED 

requirements makes determining which of these vehicles to test (and to what standards) 

administratively very difficult. Additionally, EPA's MOEILE6 emission factor model will not 

provide emission reduction credit for the performance of OED-VM checks on pre-1996 MY 

vehicles. 

EPA also does not recommend that vehicles older than MY 1994 be subjected to OEDbased VM testing, even if it is determined that the vehicle is equipped with an OED computer, 

4 
and may even have a malfunction indicator light (MIL) illuminated. The reason we do not 

recommend performing an OBD-VM scan on pre-1994 MY OED-equipped vehicles is because 

such vehicles use an earlier, non-standardized generation of OBD system (also known as OBD I). 

Due to the lack of federal standards for OBD I systems, the systems themselves tend to be 

proprietary and may not be compatible with the standardized OBD II scanners that will be used in 

most IiM programs4

• 

Elements of an OBD-VM Check 

An OBD-VM check consists of two types of examination: A visual check of the 

dashboard display function and status (also known as the MIL and/or bulb check) and an 

electronic examination of the OBD computer itself. These two examinations, taken together, 

comprise the seven step procedure outlined below. 

1) Initiate an official test by scanning or manually inputting the required vehicle and 

owner information into the reporting medium (i.e., PC-based electronic reporting 

system or manual test report). 

2) Visually examine the instrument panel to determine if the MIL illuminates briefly 

when the ignition key is turned to the "key on, engine off' (KOEO) position. A 

brief period of illumination of the MIL at start-up is normal and helps confirm the 

bulb is in proper, operating condition. This portion of the test procedure is also 

known as the "bulb check." Enter the results of the bulb check into the reporting 

medium. 

3) Locate the vehicle's data link connector (DLC) and plug a scan tool into the 

connector. While it is recommended that this step be performed with the ignition 

in the "off" position, this step can also be performed with the ignition running. 

Given the variety of locations manufacturers have chosen in practice, locating the 

DLC may well be the most time-consuming element of the inspection. We will 

discuss the issue of atypical DLC location elsewhere in this guidance. 

4) Start the vehicle's engine so that the vehicle is in the "key on, engine running" 

(KOER) condition5

. The MIL may illuminate and then extinguish during this 

phase. Continued illumination while the engine is running is cause for failure. 

4 40 CFR 85.2222 "On-board diagnostic test procedures" requires that the scan tool used for the 080-UM inspection 

be capable of communicating with the OBD system in compliance with the Society of Automotive Engineers (SAE) 

Recommended Practice J 1979. 

5 While it is possible to perform the electronic scan portion of the OBD-1/M check in the KOEO position for most 

vehicles, EPA discourages this practice because it can lead to false failures for some makes and models of vehicles (such as MY 

1996-2001 Subarus). 

5 
Also, if the MIL illuminates during this phase but was not observed in step 2, the 

vehicle should not be failed for step 2. 

5) With the scan tool in the "generic OBD" mode, follow the scan tool 

manufacturer's instructions6 to determine: 

• Vehicle readiness status7 

• MIL status (whether commanded on or off)8, and 

Diagnostic Trouble Codes (DTCs) for those vehicles with MILs 

commanded on9

• 

6) Record the results of the OBD inspection in the appropriate medium. Depending 

upon the design and feature requirements of the program, this may be an 

automated process. 

7) Without clearing DTCs or readiness codes, turn off the vehicle ignition, and then 

disconnect the scan tool 10. Clearing codes - if such is necessary- should be 

reserved for the repair portion of the program (even though in test-and-repair 

programs, the same personnel may be engaged in both activities). These codes 

(and the associated "freeze-frame" data) are important for the performance of 

proper diagnostics prior to repair. 

Although the above inspection elements are listed sequentially, current regulations do not 

specify the sequence that must be followed in performing the OBD-I/M inspection, and EPA sees 

no reason for applying a rigid sequence at this time. In some cases it may make more sense to 

6 For UM purposes, the inspectors and repair technicians should be advised to conduct the scan in "generic" mode as 

opposed to a vehicle manufacturer specific mode. EPA is aware of some instances in which using a scan tool in the vehicle 

manufacturer specific mode will result in confusing or misleading readings regarding vehicle readiness. 

7 Refer to SAE Jl979 MODE 01PIO01 DATA C and D. 

8 Refer to SAE Jl979 MODE 01PIO01 DATA A BIT 7. 

9 EPA's original OBD-1/M failure criteria were limited to power-train, emission-related DTCs (refer to SAE JI 979 

MODE 03). In its April 5, 2001 rulemaking, however, EPA simplified the failure-triggering OTC criteria to any OTC that leads 

to the MIL being commanded on. As part of its technical support efforts for the April 5, 2001 rulemaking, EPA looked at six 

months' worth ofOBD-1/M data from the Wisconsin I/M program and found that less than 0.5% of the OBD-equipped vehicles 

tested during that period experienced Mils being commanded on for DTCs falling outside the previous failure criteria. As a 

result, EPA does not believe the simplified failure criteria will result in higher overall l/M failure rates -- especially not in those 

areas that opt to replace existing tailpipe testing on MY 1996 and newer vehicles with the 080-1/M scan. 

IO For programs conducting both 080 and tailpipe testing on 080-equipped vehicles, the tailpipe test may be 

conducted prior to this step, to avoid an extra, unnecessary key-off, key-on cycle. 

6 
conduct the visual portion of the inspection after performing the onboard computer scan. For 

example, a state choosing to perform both a traditional tailpipe test and the OBD-1/M check on 

OED-equipped vehicles may choose to reduce the overall test time involved by conducting the 

OBD scan at the same time the other test is performed. EPA has found that a scan tool can be 

plugged into a still-running vehicle without producing erroneous readings. Therefore, we believe 

states should be allowed the flexibility to determine the optimum test sequence to meet their 

programmatic needs. However, EPA does caution that unforeseen problems may arise with some 

subset of the fleet due to changes in the sequence. EPA therefore asks that states consult with the 

Agency should they find unusual failure patterns among certain makes and models of vehicles in 

conjunction with the use of alternative test sequences. 

For readers who prefer their information presented graphically, a flowchart of an 

acceptable OBD system check is included in Appendix E of this guidance document. It was 

developed by the Center for Automotive Science and Technology at Weber State University, and 

is consistent with EPA guidance. 

Basis for Failure or Rejection 

Unless otherwise noted in this guidance, a vehicle should be failed for any of the 

following five reasons, with the exception of the last (for which the appropriate action is 

rejection): 11 

1) It is a 1996 or newer vehicle and the data link connector (DLC) is missing, has 

been tampered 12 with, or is otherwise inoperable. (Action: Failure) 

2) The MIL does not illuminate at all when the ignition key is turned to the KOEO 

position. The MIL should illuminate (on some vehicles, only for a brief period of 

time) when the ignition key is turned to the KOEO position. (Action: Failure) 

3) If the MIL illuminates continuously or flashes after the engine has been started, 

even if no fault codes are present, since this could indicate a serial data link 

11 States should be aware that some vehicles have atypical OBD configurations, and should take steps to avoid unfairly 

penalizing motorists. For example, states may incorrectly suspect motorist tampering for those vehicles that are manufactured 

with the DLC in a hard-to-find location. EPA is working with manufacturers, operating OBD-1/M programs, and Weber State 

University to develop an online clearinghouse ofOBD-related information useful to state UM programs and other stakeholders, 

including all OBD-related Technical Service Bulletins (TSBs) from the manufacturers and all relevant updates. See Appendices 

B through D for more information on vehicles with atypical OBD system issues. 

12 Tampering is considered to be any modification of the vehicle that deviates from the certified configuration of the 

vehicle, particularly if such modification has the practical effect of making the vehicle untestable (by, for example, making the 

DLC inaccessible) or otherwise constitutes an attempt to evade the program (by, for example, using illegal aftermarket devices 

designed to circumvent the OBD computer or provide false results during an OBD-UM check). Under this definition, moving a 

DLC as part of collision repairs would not necessarily constitute tampering -- provided the DLC was not hidden or rendered 

otherwise inaccessible as a result of being moved. 

7 
failure. 13 (Action: Failure) 

4) Any DTCs are present and the MIL status, as indicated by the scan tool, is 

commanded on, regardless of whether or not the MIL is actually illuminated. 

Do not fail the vehicle if DTCs are present and the MIL status, as indicated by 

the scan tool, is off, because such non-MIL-triggering DTCs are considered 

"pending" and frequently self clear without requiring repair of the vehicle. MIL 

command status must be determined with the engine running. (Action: Failure) 

5) The number of OBD system monitors showing a "not ready" status exceeds the 

number allowed for the model year in question. (Action: Rejection) 14 

Table 1 below lists the possible test outcomes in tabular form. 

Table 1 - Possible OBD-1/M Outcomes 

Vehicle Passes If: * Bulb check OK and 

*MIL not lit while engine running and 

* MIL not commanded on for any DTCs and 

* All required readiness codes are set 

Vehicle Fails If: * Bulb check not OK and/or 

* MIL lit while engine running and/or 

* MIL commanded on for any DTC and/or 

* DLC missing, tampered, or inoperable 

Vehicle Rejected If: * More unset readiness codes found than allowed based on MY and/or 

* DLC cannot be located or is inaccessible 

13 States should be aware that some vehicles will illuminate a MIL when a scan tool is connected and the vehicle is still 

in the "key on, engine off' condition. In some cases, the scan tool will indicate that the MIL is, in fact, commanded on -- even 

though no DTCs may be present. EPA has found that these vehicles will usually extinguish the MIL and remove the "MIL 

commanded on" indicator when the engine is started. To avoid falsely failing vehicles, therefore, it is important that the 

electronic portion of the OBD-I/M check be conducted only with the vehicle in the "key on, engine running" condition (as 

indicated in the test procedure described above). 

14 Although earlier requirements stipulated that OBD-equipped vehicles be rejected from further testing if any monitor 

was "not ready," EPA has revised these readiness criteria to allow states to not reject MY 1996-2000 vehicles with two or fewer 

unset readiness codes, or MY 2001 and newer vehicles with no more than one unset readiness code. The complete MIL check 

and scan should still be run in all cases, however, and the vehicle should still be failed if one or more DTCs are set and the MIL 

is commanded on. The vehicle should also continue to be rejected ifthe OBD computer does not set readiness codes for 3 or 

more monitors on MY 1996-2000 vehicles, or two or more monitors on MY 2001 and newer vehicles. Readiness codes in 

general, and the specific codes and conditions covered by the April 5, 2001 amendments will be discussed in more detail under a 

separate section of this guidance. 

8 
l/M-Related DTCs 

Until recently, Federal l/M regulations identified a subset of power train (or P-code) 

DTCs as being relevant for l/M purposes. If a vehicle was identified through an l/M program as 

having a MIL commanded on for one or more of those P-codes, then Federal regulations 

required that the vehicle fail the inspection. In an attempt to simplify these failure criteria - and 

to harmonize Federal requirements with California Air Resources Board (CARB) requirements 

-EPA amended this requirement as part of its April 5, 2001 rulemaking. Under the revised 

failure criteria, a vehicle shall now be failed for the presence of any DTC that results in the 

"Check Engine" MIL being commanded on. 

In commenting on an earlier draft of this implementation guidance, several commenters 

raised concerns that EPA' s simplified failure criteria would result in vehicles being failed for 

non-emission related components or systems, such as the brakes or suspension. Although some 

vehicle onboard computers may monitor non-emission-related components and systems at the 

manufacturer's discretion, Federal regulations require that the "Check Engine" MIL only be 

illuminated for emission-related malfunctions. Other dashboard lights may be illuminated to 

indicate the need for service of a non-emission-related component or system, but the presence 

of such lights does not constitute grounds for failing the OBD-1/M check. Furthermore, EPA 

has examined data from over 300,000 OBD-1/M checks performed in actual l/M lanes and has 

not found a single instance of the simplified failure criteria leading to the failure of a vehicle for 

a non-emission-related component or system. 

EPA also wants to acknowledge that it is possible we may need to limit the criteria for 

failing OBD-equipped vehicles after such vehicles reach an as-yet undetermined age and/or 

mileage. The reason for considering this possibility stems from the fundamental difference 

between how OBD triggers repairs versus how traditional tailpipe tests trigger repairs. 

Traditional l/M tailpipe tests identify a vehicle as failing for a given pollutant through direct 

sampling of the exhaust plume. These tests vary in the degree to which they provide any 

additional information that can be used to target the component or system failure that has led to 

the high emission reading. In such programs, repair technicians have a fair degree of discretion 

when it comes to recommending repairs to address a given failure, although owners are 

protected from excessive economic hardship by the cost waiver option. OBD, on the other 

hand, identifies specific components and/or systems in need of repair or replacement. As a 

result, EPA foresees the possibility that some advanced-aged OBD-equipped vehicles could be 

failed for DTCs for which the only available repair option would cost substantially more than 

the fair market value of the vehicle itself. Under such a scenario, the waiver option does not 

offer much consumer protection, since such repairs tend to be all-or-nothing propositions. For 

example, a motorist faced with a transmission repair cannot reasonably opt to have the 

transmission "half fixed" to take advantage of the cost waiver option. 

Given the relative newness of OBD II, EPA has not been able to gather the data 

9 
necessary to determine whether situations like the one above will actually happen in practice 

(though the Agency certainly plans to gather such data in the future). We do believe however 

that program requirements should be reasonable, and that the economic burdens of a program 

should be balanced by the environmental benefit likely to result from the imposition of those 

burdens. Therefore, we may revise our failure criteria at some future date, once data has been 

gathered and analyzed concerning the actual costs associated with repairing high mileage/age 

OED-equipped vehicles across the full range of possible MIL-triggering DTCs. 

Test Report 

If a vehicle fails, the test report given to the motorist should include the status of the 

MIL illumination command and the alphanumeric fault code(s) listed along with the DTC 

definition(s) as specified per SAE 12012and11930. Only the fault codes leading to the 

inspection failure should be listed on the report given to the motorist. EPA makes this 

recommendation because it is possible that an OBD system may set DTCs without commanding 

a MIL to be illuminated. These DTCs usually reflect an intermittent condition which may or 

may not be a problem at the time of testing. If the condition does not recur within a certain 

number of trips, the code will eventually be cleared; if the condition does recur, the system may 

then determine that a MIL should be illuminated. Therefore, no DTCs should be printed on test 

reports for vehicles that pass the inspection. An owner who receives notice of these codes on 

the same sheet of paper with notification of passing the state inspection may become confused 

or desensitized to the importance of DTCs and the MIL. Lastly, unset readiness codes should 

also be listed on the report if the number of unset readiness codes exceeds the limit for which 

an exemption is allowed (i.e., ifthe outcome of the test is rejection based upon the presence of 

too many unset readiness codes). If the number of unset readiness codes falls below the limit 

for which an exemption is allowed (and the vehicle would otherwise pass the inspection) then 

no unset readiness codes should be listed on the test report provided to the motorist. 

Readiness Status: Initial Test 

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 unique 

to each manufacturer and involve such things as ambient temperature as well as driving 

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 

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 monitor in question has been evaluated), 

10 
"not ready" (meaning the monitor has not yet been evaluated), or "not applicable" (meaning the 

vehicle is not equipped with the component monitor in question). 

There are several reasons why a vehicle may arrive for testing without the required 

readiness codes set. These reasons include the following: 

1) Failure to operate the vehicle under the conditions necessary to evaluate the 

monitor(s) in question; 

2) A recent resetting of the OBD system due to battery disconnection or replacement, 

or routine maintenance immediately prior to testing15 ; 

3) A unique, vehicle-specific OBD system failure; 

4) An as-of-yet undefined system design anomaly; or 

5) A fraudulent attempt to avoid IfM program requirements by clearing OBD codes just 

prior to OBD-J/M testing (by, for example, temporarily disconnecting the battery). 

In addition to the above considerations, EPA has also found that a small number of 

vehicles may be flagged as "not ready" or "not supported" for one or more of the continuous 

monitors (i.e., misfire, fuel trim, and/or comprehensive components). This makes no sense 

because continuous monitors are designed to run continuously (as their name implies) and 

therefore should always be flagged as "ready." In its investigation of this issue, EPA has 

determined that the problem is the result of incompatibility between the vehicle and scanner 

software and is not indicative of a fault with the vehicle's OBD system. As a result of this 

discovery, EPA recommends that programs disregard these continuous monitors when 

establishing the readiness status of the vehicle. This exclusion is for readiness determination 

purposes only; a vehicle with a MIL commanded on for a continuous monitor based DTC 

should continue to be failed in compliance with the test procedure discussed earlier in this 

guidance. EPA is working with state programs and OBD software suppliers to address this 

issue and will issue revised guidance as warranted. 

Because the presence of unset readiness codes among the non-continuous monitors 

could be a sign of attempted fraud, it is important that all OED-equipped vehicles be checked to 

confirm that readiness codes have been set as one of the pre-requisites for a valid OBD-I/M 

inspection. Nevertheless, as described in the FRM, EPA also believes that the previous 

requirement regarding readiness codes (i.e., that a vehicle be rejected from further testing if any 

monitor is found to be "not ready") was more rigorous than either necessary or practical. 

Therefore, as discussed under "Basis for Failure or Rejection" above, EPA has revised the 

15 As part ofa program's outreach effort, car owners should be advised to allow for approximately one week of normal 

vehicle operation after repairs and prior to OBD-UM testing (or retesting). 

11 
readiness requirement so as 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 completed provided there is no more than one unset readiness code. This 

does not mean that these vehicles are exempt from the OBD-I/M check16• The complete MIL 

check and scan must be run in all cases, and the vehicle still must be failed if any of the failure 

criteria discussed in this guidance are met. The vehicle should continue to be rejected if it is 

MY 1996-2000 and has three or more unset, non-continuous readiness codes or is MY 2001 or 

newer and has two or more unset, non-continuous readiness codes. 

As discussed in the Technical Support Document for the FRM, this amendment is based 

upon EPA's findings regarding readiness codes from Wisconsin's OBD-1/M data and also 

reflects a FACA workgroup recommendation. Since August 1998, Wisconsin's l/M program 

contractor has been sending to EPA OBD scanning and IM240 test results data collected on 

MY 1996 and newer vehicles coming through the Wisconsin I/M test lanes. In analyzing the 

Wisconsin data, EPA made the following observations regarding the readiness status of the 

OED-equipped vehicles presented for testing17 : 

• The majority of vehicles showing up at the I/M lane with readiness codes reading 

"not ready" were from MY 1996; the "not ready" rate for MY 1996 vehicles was 

5.8%. 

• The frequency of vehicles with readiness codes reading "not ready" dropped off with 

each successive model year - to 2.2% for MY 1997 and 1.4% for MY 1998. 

• If an exemption were allowed for up to two readiness codes to read "not ready" 

before a vehicle would be rejected from further testing, the rejection rate drops - to 

2.2% for MY 1996 and to 0.2% for MY 1997 and MY 1998, for a three model year 

average of 0.9%. 

The intention behind EPA's decision to allow limited exemptions from the readiness 

16 As discussed above, when determining the readiness status ofa vehicle, EPA recommends that programs only 

consider the non-continuous monitors. Likewise, in assessing whether a "not ready" vehicle can be exempt from the readiness 

rejection requirement based upon the number of unset readiness codes present, only unset readiness codes among the noncontinuous monitors should be counted. 

17 In commenting on an earlier draft of this implementation guidance, some commenters suggested that EPA's figures 

regarding the frequency of"not ready" vehicles in the in-use fleet are skewed downward because the Agency's data comes from a 

program where the OBD-1/M check is being performed on an "advisory only'' basis. These commenters argued that in an 

"advisory only" program motorists had no incentive to fraudulently clear M!Ls by disconnecting the battery since there was no 

negative repercussion from showing up for the inspection with a lit MIL. These commenters further argued that once motorists 

began failing based upon M!Ls, the frequency of vehicles with unset readiness codes at the time of the initial test would go up. 

To test this hypothesis, EPA has begun analyzing program data from the Oregon OBD-1/M program, which is currently failing 

vehicles based upon the results of the OBD-1/M check. Based upon a preliminary analysis of the Oregon data, EPA has found 

that roughly 1.4% of MY! 996+ vehicles are being rejected at the time of the initial test due to an excessive number of"not 

ready" codes (assuming the readiness exemption allowance provided by the April 5, 2001 FRM). 

12 
rejection criteria is exclusively to avoid inconveniencing motorists on the basis of vehicle 

conditions that are beyond their control, that are currently the subject of discussion between 

EPA and various manufacturers, and that, in some cases, may result in potential enforcement 

action. The purpose of the limited readiness exemption is not to relieve manufacturers of their 

responsibility to design and market OBD systems that comply with existing OBD certification 

requirements 18

• Nothing in this guidance in any way changes or otherwise impacts these 

obligations on the part of vehicle manufacturers. In fact, EPA has already initiated several 

investigations which may result in enforcement actions related to these requirements. 

Because of the small number of vehicles involved, EPA believes that the environmental 

impact of the limited readiness exemption will be negligible, especially given the likelihood 

that at least some of these readiness codes will have been set in time for subsequent OBD-VM 

checks, and the fact that an unset readiness code is not itself an indication of an emission 

problem. EPA believes that allowing limited exemptions from the readiness code requirement 

as described above makes the most sense at this time, while EPA, CARB, and the 

manufacturers work to clarify system function requirements with regard to VM. Lastly, EPA 

does not believe that allowing these limited exemptions will interfere with the use of readiness 

codes to help deter possible fraud because such fraud would inevitably lead to more monitors 

being set to "not ready" than are allowed under EPA's limited exemptions. 

In addition to the above exemptions, EPA also recommends that VM programs waive 

the readiness requirement or otherwise accommodate specific makes, models, and model years 

of vehicles with known readiness design problems, in accordance with applicable technical 

service bulletins and/or EPA guidance. EPA has compiled a list of such vehicles and included 

it in Appendix D. 

Even with these vehicle-specific accommodations and the above exemptions, however, 

some vehicles will still need to be rejected based upon readiness code status. In the case of a 

vehicle rejected for unset readiness codes (which does not otherwise meet the failure criteria 

described in this guidance), the motorist should be given the option of operating the vehicle for 

a week under normal operating conditions in an attempt to evaluate the necessary monitors 

without being required to visit a repair facility prior to retesting. If the monitors still have not 

performed an evaluation by the first retest, the motorist should then be advised to visit a repair 

18 To help emphasize this point, EPA clarifies here that the obligations of the automobile manufacturers with regard to 

OBD equipment are specified in regulatory section 40 CFR 86.094-l 7(e)(l): "Control of Air Pollution From New Motor 

Vehicles and New Motor Vehicle Engines: Regulations Requiring On-Board Diagnostic Systems on 1994 and Later Model Year 

Light-Duty Vehicles and Light-Duty Trucks," which imposes, among other things, the obligation to design, build and certify 

OBD systems that: "record code(s) indicating the status of the emission control system. Absent the presence of any fault codes, 

separate status codes shall be used to identify correctly functioning emission control systems and those emission control systems 

which need further vehicle operation to be fully evaluated." In promulgating these requirements on February 19, 1993 the 

Agency stated: 'The readiness code will ensure l/M testing personnel and service technicians that malfunction codes have not 

been cleared since the last OBD check of the vehicle's emission-related control systems. This code will be essential ... since l/M 

personnel must be sure that the OBD system has sufficient time to completely check all components and systems. The readiness 

code is also crucial for indicating to service personnel whether any repairs have been conducted properly." 

13 
facility where the monitors can be set based upon vehicle-specific, manufacturer guidance. 

Alternatively, states may decide to allow such vehicles to default back to the traditional I/M 

tests performed on the vehicles in question prior to introduction of the OBD-VM test 

requirement. EPA recommends that states that choose to use back-up tailpipe testing in lieu of 

rejection track this activity carefully to ensure that the practice does not lead to an inadvertent 

loophole through which motorists routinely avoid the OBD-1/M inspection in favor of the 

tailpipe test. 

In all cases, it is important to emphasize that lack of readiness is a special status 

particular to OBD systems and that the vehicle is not necessarily producing excess emissions. 

Instead, the vehicle's emissions status is officially "Unknown," due to a failure to meet 

certain monitoring conditions prior to the inspection. In order to minimize confusion, EPA 

recommends that states provide a written statement about OBD and readiness status to 

motorists who are rejected based upon an excessive number of unset readiness codes. Such a 

statement should make the following key points: 

1) A vehicle rejected as "not ready" is not necessarily "dirty." 

2) "Not ready" just means the vehicle's computer has not had an opportunity to fully 

evaluate the vehicle's performance. 

3) Many circumstances can lead to a vehicle being "not ready," including recent 

vehicle repairs and/or battery replacement. 

4) In most cases, a week's worth of continued vehicle operation under normal 

operating conditions will be sufficient to make a "not ready" vehicle "ready." 

5) In a very limited number of cases (less than 1 %), a "not ready" vehicle may need 

to be taken to a repair facility, where the readiness codes can be set based upon 

vehicle-specific, manufacturer guidance. 

Readiness Status: Retest After Repairs -- Non-catalyst-related DTCs 

OBD-1/M programs also must address the readiness code status of vehicles returning for 

retesting after repairs have been performed to correct an initial OBD-VM failure. Even if the 

vehicle showed all readiness codes as "ready" on the initial test, vehicles returning to the I/M 

lane immediately following repair will likely have just had the fault code memory cleared by 

the repair technician (the proper step following a repair). Upon clearing the fault code memory, 

however, all readiness codes will also be cleared and set to "not ready." If the vehicle returns 

for retesting immediately after repair, it is possible that one or more readiness codes will 

register as "not ready." To address this possibility, EPA recommends that the vehicle be held to 

the same readiness criteria as are applicable for an initial test (i.e., if the vehicle is MY 1996-

14 
2000, a maximum of any two unset readiness codes may be allowed, while for MY 2001 and 

newer vehicles, no more than one unset readiness code is allowed). 

To help minimize the potential for vehicles showing up for their retest with an excessive 

number of unset readiness codes, outreach to the repair community should stress the importance 

of confirming vehicle readiness prior to returning a repaired, OED-equipped vehicle to its 

owner. Motorists should also be informed that they should plan to allow a week's worth of 

ordinary driving between receiving repairs and getting a vehicle retested, to avoid being 

rejected based upon an excessive number of unset readiness codes. If, despite these caveats, a 

vehicle is presented for retesting with an excessive number of unset readiness codes after repair, 

EPA believes that the submission of repair receipts as proof of repair is an adequate method for 

establishing that the necessary repairs have been performed. EPA appreciates that the ability of 

inspectors to confirm repairs prior to retesting will vary, depending upon whether the VM 

program is a test-and-repair or test-only program. In the case of test-only programs, outreach 

efforts to the repair community should stress the importance of including an indication on the 

repair receipt that the repairs in question are OED-related (i.e., by including the diagnostic scan 

in an itemized list of services performed). A repair receipt (as opposed to a repair estimate) 

including evidence of a diagnostic scan and dated either on the same day as the initial test or 

sometime thereafter may be considered adequate for establishing proof-of-repair for retest 

purposes in test-only programs. In the case of owner-performed repairs, the program should 

require the submission of appropriately dated parts receipts prior to retesting, and these receipts 

should be reviewed by the test station manager, who in tum should be trained to determine 

whether the parts in question are relevant to the cause of failure. EPA believes that the number 

of vehicles falling into this last category (i.e., OED-equipped vehicles that fail the initial test 

and return for retesting with owner-performed repairs and an excessive number of unset 

readiness codes) should be relatively small 1

9

• 

In commenting on an earlier draft of this implementation guidance, some commenters 

raised concern that it would be possible for repair technicians to selectively clear DTCs without 

performing repairs and without setting the remaining monitors (i.e., those without DTCs 

recorded) to "not ready." If this were possible -- the commenters argued -- then vehicles that 

should be failed could be fraudulently passed on the retest (even without receiving repairs) 

because of the readiness exemptions allowed by EPA (assuming the number of DTCs resulting 

in the initial failure did not exceed the number of readiness exemptions allowed for the model 

year in question). In reality, it is not possible to selectively clear DTCs or to only set some 

readiness monitors to "not ready" while leaving the remaining monitors "ready." As currently 

designed, the feature which allows the clearing of DTCs is an all-or-nothing proposition --

19 In looking at data from the Oregon OBD-1/M program, EPA has found that for the tailpipe test, the average number 

of retests required before passing after the initial failure is 2.01 , while for vehicles failing the OBD-1/M check, the average 

number ofretests required before passing after the initial failure is 1.39 (even after accounting for the retest rejection rate 

associated with the readiness issue described in this section). 

15 
specifically to avoid fraud such as that suggested by the commenters20• 

Readiness Status: Retest After Repairs -- Catalyst-related DTCs (P0420 - P0439) 

Based upon an analysis of data from the Oregon OBD-1/M program EPA recommends 

that vehicles which fail the initial OBD test for any of the catalyst monitoring codes (P0420 

through P0439) be held to a higher standard for the retest than is the case with other failure 

codes. EPA recommends that initial catalyst failures follow one of the following steps at the 

time of retest: 

1) If the catalyst monitor is "ready" at the time of retest, then the vehicle should be 

treated like any other vehicle returning for retest after failing for any non-catalystrelated DTC. 

2) If the catalyst monitor is "not ready" at the time of retest, then the owner should be 

required to provide proof of repair, or the vehicle should be required to pass a 

tailpipe test to verify catalyst function. 

3) Alternatively, at the program's discretion, if the catalyst monitor is "not ready" at 

the time of retest, the vehicle should be rejected until the catalyst monitor has been 

set to "ready." 

Readiness Status: Continuous Monitors 

As previously mentioned, EPA has found that a small number of vehicles may be 

flagged as "not ready" or "not supported" for one or more of the continuous monitors (i.e., 

misfire, fuel trim, and/or comprehensive components). EPA recommends that programs 

exclude these continuous monitors from consideration when establishing the initial readiness 

status of the vehicle. This exclusion is for readiness determination purposes only; a vehicle 

with a MIL commanded on for continuous monitor based DTCs should continue to be failed in 

compliance with the test procedure discussed earlier in this guidance. EPA is working with 

state programs and OBD software suppliers to address this issue and will provide revised 

guidance as warranted. 

In the case of vehicles which fail the initial OBD-1/M check exclusively for DTCs 

related to the continuous monitors, repair technicians should be instructed to not clear the DTCs 

electronically after performing the necessary repairs. Instead, the success of repairs based upon 

continuous monitor related DTCs should be confirmed by letting the OBD system detect the 

repair and reset itself to "ready" (a process which should occur naturally after 3 engine key20 Refer to SAE J 1979 - "E/E Diagnostic Test Modes," Mode $04. 

16 
on/key-off cycles, provided the repairs were performed correctly). In the case of initial failures 

for a mix of continuous and non-continuous monitor related DTCs, the DTCs should continue 

to be cleared, post-repair, as recommended elsewhere in this guidance. 

Evaporative System Testing and OBD 

EPA's analysis of the Wisconsin VM lane data suggests that OBD-I/M testing can be 

supplemented by including a separate gas-cap check. When EPA compared failure rates for the 

evaporative portion of the OBD-VM test to the failure rate for the stand-alone gas cap test we 

found that the separate gas cap test was able to identify a substantial number of leaking gas caps 

that were not identified by the OBD monitors due to the different failure thresholds. 

The seeming disparity described above is a result of the different detection thresholds 

for the two tests. The stand-alone gas cap test was designed to detect a leak as small as 60 

cubic centimeters per minute (cc/min) at a pressure of 30 inches of water, while OBD systems 

were designed to detect leaks equal to a circular hole 0.040 inches in diameter. The 0.040 inch 

hole equates to a flow rate in excess of 2,600 cc/min at 10 inches of water column (i.e., the 

maximum allowable internal tank pressure using the enhanced evaporative emission test)21 • As 

a result, an OBD system can reliably detect a loose or missing gas cap, while a properly 

tightened but leaking gas cap that can easily be identified by the gas cap test will probably not 

be identified by OBD. 

Since the gas cap test is able to identify an excessive emission condition not identified 

by OBD, EPA recommends including this additional testing element in those areas that need 

substantial reductions in hydrocarbon (HC) emissions from mobile sources as part of their 

ozone attainment plans. For states with more modest air quality needs with regard to mobile 

sources, EPA is leaving it to the states to assess their needs regarding whether or not gas cap 

testing is added to the OBD testing regime. EPA is reserving judgement at this time because 

we still do not have sufficient data to draw reliable conclusions concerning the frequency of 

leaking gas caps in the in-use fleet. Our efforts in these areas have been complicated as a result 

of pre-inspection replacement of the gas cap and, in some cases, a failure by inspectors to 

record the initial gas cap failure as a failure. During informal audits of such programs, EPA has 

found that the faulty gas cap is frequently replaced on the spot, or the owner is directed to 

simply replace the cap later without being required to return for a retest22

• 

21 During its pilot testing ofOBD evaporative monitor effectiveness, EPA found that some in-use OBD systems were 

capable of detecting leaks from holes as small as 0.020 inches in diameter. A 0.020 inch hole equates to 600 cc/min at 10 inches 

of water column pressure. 

22 EPA encourages states conducting the gas cap pressure check to stress the importance of performing and recording 

the results of the gas cap test accurately as part of their on-going outreach, training, and enforcement activities. 

17 
Implementation Issues for Centralized vs. Decentralized OBD-Based Testing 

While EPA recommends that l/M programs integrate OBD-1/M test procedures into 

an overall, PC-based, real-time data-linked testing system, we understand that some programs 

which do not currently require test stations to be linked to a real-time database may opt to use 

generic, stand-alone, handheld scanners that do not generate automatic test reports and are 

not tied to a real-time data-linked system. While the use of stand-alone scanners is not barred 

by l/M regulations for those areas not otherwise required to employ a real-time database, 

EPA nevertheless sees several drawbacks to the stand-alone approach to OBD-1/M testing. 

For example, the lack of a real-time data link will mean that program oversight will 

necessarily be more costly, more labor intensive, and also less comprehensive, leaving the 

program perhaps more vulnerable to fraud. This decrease in program oversight effectiveness 

would come at a time when a significant portion of the program itself is reverting to what is, 

in effect, a manual test program, where test reports are filled out by hand from information 

read off a handheld scanner's screen. Historically, programs that rely upon a non-automated 

process for making pass/fail decisions have been found to be even more difficult to oversee 

than traditional decentralized programs, since no electronic record is produced, making 

auditing more difficult. Furthermore, the use of computer matching to identify noncomplying vehicles would be seriously restricted under such a system, assuming that such a 

system would not result in an electronically searchable testing database. And while it is 

possible that manually-completed test reports could be made computer-scannable and 

collected during site visits or sent to the state, the inherent time lag between the test and 

inclusion in the state database makes this a challenging implementation issue (i.e., a negative 

hit could equal data lag, not necessarily non-compliance). Lastly, the individual station's 

access to extensive and important program information (for example, DLC location databases 

and technical service bulletins regarding program updates, pattern failures, etc.) would be 

limited, both in terms of availability and timeliness. 

At a minimum, EPA believes that for an OBD-1/M test program to be most effective -

- whether centralized or decentralized -- it should be designed in such a way as to allow for: 

Real-time data link connection to a centralized testing database; 

Quality-controlled input of vehicle and owner identification information 

(preferably automated, for example, through the use of bar code); and 

Automated generation of test reports. 

OBD and Inspector Fraud 

As is the case with all other l/M test types, the OBD-1/M check is vulnerable to 

inspector fraud, and program managers need to be on guard to limit the opportunities for this 

18 
kind of activity. For example, it is currently possible for an unscrupulous inspector in a 

tailpipe-based program to engage in a practice known as "clean piping," where a knownclean vehicle is tested while the vehicle identification information for another (presumably 

dirty) vehicle is entered into the test record. Similarly, there is a limited opportunity for an 

inspector to "clean scan" an OED-equipped vehicle, but there are also methods for keeping 

this type of activity in check. The opportunity for "clean scanning" exists because the vehicle 

identification number (VIN) is not currently included in the data stored in the vehicle's 

onboard computer. Unlike "clean piping," however (where almost any known-clean vehicle 

will do), the opportunity for large-scale "clean scanning" can be greatly reduced through the 

use of identity-limiting information which is currently available from the vehicle's OED 

system. For example, programs could tally the number of Parameter Identifications (PIDs) 23 

supported by the vehicle, which can be used as a check against the other vehicle information 

entered into the test record. Another important number to capture and track for quality 

control purposes is the Powertrain Control Module (PCM) diagnostic address24• While these 

numbers do not identify a vehicle down to the level of an individual registration and owner, 

they do allow for the separation of vehicles into different makes, models, and engine families. 

Put another way, the PID count and PCM diagnostic address for a Honda Accord will be 

different from that of a Ford Escort. Therefore, programs can limit the potential for fraud via 

"clean scanning" by comparing the PID count and/or the PCM diagnostic address to the other 

vehicle information in the test record. EPA is working with manufacturers and states 

currently implementing the OED-1/M inspection to gather the data necessary to interpret PID 

count and PCM diagnostic address information so it can be used for this purpose. 

In commenting on an earlier draft of this implementation guidance, some commenters 

suggested that even though the use of PID counts and PCM diagnostic addresses could limit 

the potential for fraud via "clean scanning" among garages and service stations, it does not 

pose much of a deterrent for dealerships, which have a readily available supply of vehicles of 

the same make and model as vehicles being tested. While this may be the case, EPA does not 

believe that the potential for fraud among OED-equipped vehicles is any higher than the 

current potential for fraud via clean piping. Furthermore, if a state is concerned that 

dealerships pose a greater fraud threat than other service providers, the state certainly has the 

discretion to monitor the compliance of those dealerships and take appropriate enforcement 

action, should fraud be detected. 

Repair Cost Waivers and OED 

Though for equity reasons it may be difficult for states to eliminate the waiver option 

for OED-tested vehicles, EPA recommends that states consider at least modifying waiver 

23 Refer to SAEJl979 MODE 01PID00. 

24 Refer to SAE J 1979 Section 4.2.4 Header Bytes. 

19 
requirements for such vehicles. The reason for wanting to avoid granting a waiver to a 

vehicle with an illuminated MIL is two-fold: 1) it reinforces bad behavior (i.e., ignoring 

illuminated MILs) and 2) once lit, a MIL that was illuminated for a relatively minor problem 

effectively eclipses new, major problems, should they develop. At a minimum, the state's 

public education efforts regarding the OBD inspection should stress the importance of 

responding to illuminated MILs in a timely manner. 

Public Outreach 

In recognition of the pivotal role repair technicians and the public play in the success 

of l/M programs, EPA recommends that all states required to perform vehicle OBD system 

checks begin public outreach and technician training six months to a year prior to the 

beginning of mandatory OBD testing25 • Therefore, another reason for issuing this guidance at 

this time is to give states the opportunity to consider the various issues raised by and 

addressed in this guidance in the development of their public outreach and technician training 

efforts. The need for public outreach is also one of the reasons EPA has provided states 

several options for postponing the deadline for mandatory OBD testing beyond January 1, 

2001 as part of its April 5, 2001 rulemaking. 

To facilitate a smooth incorporation of OBD-based testing of OED-equipped vehicles 

into l/M programs states should not underestimate the importance of effective public 

outreach campaigns to inform motorists and the repair community about OBD and how it 

works, what the MIL is and how to respond to it, and the environmental and consumer 

benefits of OBD. Thorough explanation of the OBD system within the context of I/M testing 

may guard against the negative public perception which accompanied the introduction of 

loaded mode testing in many areas. Extra care may need to be taken in areas where loaded 

mode testing made the state emissions testing a "hot button" issue. 

Once developed, public educational materials should be disseminated as widely as 

possible. Relevant distribution points include: Trade organizations, dealerships (service 

writers as well as technicians), AAA and other insurance-provider newsletters, private 

garages, owners manuals for MY 1996 and newer vehicles, EPA publications, auto shows, 

drive-time radio advertisements, automotive magazines, and environmental public service 

announcements. In pursuing their public outreach efforts, states should be sure to involve all 

relevant parties in the process of developing and distributing materials. These include: State 

25 EPA also recommends that states consider factoring in a month or more of voluntary, advisory-only testing to allow 

inspectors and motorists to get accustomed to the program and to allow for debugging prior to the beginning of mandatory testing 

with mandatory repairs upon failure. Under such a scenario, a vehicle would be given a complete OBD-1/M test with the 

exception that vehicles would not be failed on the basis of the OBD-1/M check alone, and would, instead, be issued an advisory 

notice indicating that the vehicle is experiencing a problem for which it will be failed if corrective action is not taken prior to the 

next test cycle. Under the one-test-cycle phase-in option allowed by the April 5, 2001 FRM, vehicles which fail the OBD-1/M 

check but pass the tailpipe inspection should be provided an advisory such as the one described above if they are excused from 

being repaired during the phase-in period. 

20 
legislators, local leaders, automobile manufacturers, automobile enthusiasts, scan tool 

manufacturers, EPA regional offices, emission inspection contractors, environmentalists, 

health professionals, AAA and other automotive insurance providers, technical colleges, 

service writers, private garages and repair chains. 

Technician Training 

The success of a state's OED-I/M effort will also depend on making sure that the 

repair community is prepared to address the sorts of vehicles that are identified by the OED 

scan as needing service and/or repair. States should work with their local educational 

institutions, OED equipment vendors, and other training providers to ensure that the 

necessary training is available to repair technicians in the field well in advance of mandatory 

OED-I/M testing. In addition, states should also work with the various organizations 

representing the repair community to stress the need for repair technicians to take advantage 

of the training opportunities that are available. 

In I/M programs where repair technicians are licensed or certified by the state to 

participate in the program, OED-specific repair technician training should be required as a 

prerequisite to such licensing or certification. Such training should address the following 

topics, at a minimum: 

• The basics of OED (i.e., theory, terminology, legal requirements, etc.) 

• The differences between OED I and OED II 

• The OED-I/M inspection procedure 

• The pass, fail, and rejection criteria for OED-equipped vehicles 

• Readiness, the setting and clearing of codes, and MIL-triggering vs. pending 

DTCs 

• The link between the OED-I/M check and the environment, and 

• Proper diagnostic procedures and available sources of diagnostic materials 

(i.e., manufacturers, hotlines, web sites, etc.). 

21 
APPENDIX A 

Glossary of 1/M- and ORD-Related Terms 

Basic I/M: A vehicle inspection and maintenance program designed to meet the basic I/M 

performance standard which includes performance of an idle test on 1968+ passenger cars. 

Under the 1990 Amendments to the Clean Air Act, basic I/Mis required in moderate 

nonattainment areas, as well as those areas already implementing or required to implement a 

basic I/M program prior to passage of the 1990 Amendments. 

"Check Engine" Light: See the definition for Malfunction Indicator Light (MIL) below. 

Diagnostic Trouble Codes CDTCs): An alphanumeric code which is set in a vehicle's onboard 

computer when a monitor detects a condition likely to lead to (or has already produced) a 

component or system failure, or otherwise contribute to exceeding emissions standards by 1.5 

times the certification standard. 

Enhanced l/M: A vehicle inspection and maintenance program designed to meet one of three 

enhanced l/M performance standards - high, low, and ozone transport region (OTR) low. 

The high enhanced standard is designed around IM240 tailpipe testing and purge and 

pressure evaporative system testing. The low enhanced standard is similar to the basic l/M 

performance standard, but includes light-duty trucks and a visual antitampering inspection. 

The OTR low enhanced performance standard is designed for areas which would not be 

required to do I/M at all, save for their location within the Northeast Ozone Transport 

Region. The OTR low enhanced standard is based upon tailpipe testing using remote sensing 

devices and visual antitampering inspections. Serious and worse nonattainment areas are 

required to implement enhanced I/M, as well as all areas within the OTR with populations 

over 100,000, regardless of attainment status. 

Evaporative System Test: A test of a vehicle's evaporative control system to determine if the 

system is 1) leaking and/or 2) purging properly. 

Malfunction Indicator Light (MIL): Also known as a Check Engine light, the Malfunction 

Indicator Light of MIL is illuminated on the dashboard when conditions exist likely to result 

in emissions exceeding standards by 1.5 times or worse. Alternatives include "Service 

Engine Soon," as well as an unlabeled picture of an engine. 

Onboard Diagnostics COBO): A system of vehicle component and condition monitors 

controlled by a central, onboard computer running software designed to signal the motorist 

when conditions exist which could lead to a vehicle's exceeding its emission standards by 1.5 

times the standard. 

OBD Data Link Connector (DLC): The interface - usually located under the dashboard on 

the driver's side - between a vehicle's OBD computer and the OBD scanner. Connecting an 
OBD scanner to the DLC allows VM inspectors and vehicle repair technicians to read the 

readiness status of the vehicle's various onboard monitors as well as any diagnostic trouble 

codes (DTCs). 

Readiness Code: A status flag stored by a vehicle's onboard computer which is different from 

a DTC in that it does not indicate a vehicle fault, but rather whether or not a given monitor 

has been run (i.e., whether or not the component or system in question has been checked to 

determine if it is functioning properly). 

Scanner or Scan Tool: A PC-based or handheld device used to interface with a vehicle's 

onboard computer for the purpose of reading DTCs and monitor readiness status. 

Test-and-Repair: An VM program which allows the same people who test a vehicle to also 

repair the same vehicle and retest it to determine whether or not the repairs performed were 

adequate. Test-and-repair programs are also generally decentralized, though not all 

decentralized programs are necessarily test-and-repair. 

Test-Only: An I/M program - usually, though not exclusively centralized - which requires 

that the functions of testing and repair be performed by different, financially unrelated 

parties. 
APPENDIXB 

Data Link Connector Mapping Diagram 
I I 

I I 

Diagnostic Link Connector Mapping Diagram 

Driver Side 

I 

I / 

I f 

\ 

I ' 

#7 

I 

I 

I 

/_ - - - -

I l Pref erred Connector Location I 

I 

I 

I 

I 

I 

\ 

\ 

\ 

\ 

Passenger Side 

• I 

I I 

--- -----.-.. __ ...... '' ' \ 

........ '":;.. ...._, \ \ ..,_..._I 

''· ) I 

/ I 

-- _.,,"" I --=::: .... --.z:-.::::= ____ ~ 

------------·--· 

t -Vehicle Centerline 
Diagnostic Link Connector (DLC) Mapping Diagram Explanation 

The mapping diagram of DLC locations contains a divided instrument panel (IP) with 

numbered areas. Each numbered area represents specific sections of the IP where 

manufacturers may have located DLCs. This document briefly clarifies the numbered 

locations on the mapping diagram. Areas 1-3 fall within the preferred DLC location while 

the remaining areas, 4-8, fall into the allowable DLC location according to EPA 

requirements. Areas 4-8 require that manufacturers label the vehicle in the preferred location 

to notify parties of the alternate connector location. 

Preferred Location(s) 

Location #1: 

This location represents a DLC positioned on the underside of the IP directly under the 

steering column (or approximately 150mm left or right of the steering column). Visualizing 

the underside of an IP divided into three equal parts from inside the passenger compartment, 

this represents the center section. 

Location #2 

This location represents a DLC positioned on the underside of the IP between the steering 

column and the driver's side passenger door. Visualizing the underside of an IP divided into 

three equal parts from inside the passenger compartment, this represents the left section. 

Location #3 

This location represents a DLC positioned on the underside of the IP between the steering 

column and the center console. Visualizing the underside of an IP divided into three equal 

parts from inside the passenger compartment, this represents the right section. 

Allowable Location(s) 

Location #4 

This location represents a DLC positioned on the upper part of the IP between the steering 

column and the center console (but not on the center console, see location #6). 

Location #5 

This location represents a DLC positioned on the upper part of the IP between the steering 

column and the driver side, passenger door. 

Location #6 

This location represents a DLC positioned on the vertical section of the center console and 

left of the vehicle center line. 

Location #7 
This location represents a DLC positioned 300 mm right of the vehicle centerline either on 

the vertical section of the center console or on the passenger side of the vehicle. 

Location #8 

This location represents a DLC positioned on the horizontal section of the center console 

either left or right of the vehicle center line. This does not include the horizontal section of 

the center console that extends into the rear passenger area (see location #9). 

Location #9 

This location, not shown, represents any DLC positioned in an area other than those 

mentioned above (e.g., in the rear passenger area on the driver side armrest). 
APPENDIXC 

Vehicles with Hard-to-Find Data Link Connector Locations*, by Make and Model Year 

(*Location numbers refer to DLC map in Appendix B) 
Sheet1 

Manufacturer I Year Model LocaUon/ i Comments ! I Access I I 

Audi 1996, 1997 Cabriolet, A6 9/cover rear ashtray ; 

Bentley 1996-2000 au 9/cover in glove box 

BMW 1999-2000 3 Series 2/cover 1/4 tum slot head screw 

BMW 1996-1998 3 Series (including '96·'99 M3) 2/cover 1 /4 tum slot head screw 

BMW 1996-2000 s Series 2/cover 1 /4 tum slot head screw - · BMW 1996. 2000 7-seri~ 6/cover under stero cntr1 ---· BMW 1996-2000 X3/M Roadsi;;,r 7/cover passenger side of c;onsole ' 

BMW 1996- 2000 Z3-series 9/cover underpassgr.dash 

Ferrari 1996-2000 all 3/open up high under the dash board 

Ford 1996 Bronco 7/cover 

Ford 1996 F Series 7/cover 

Ford 1996, 1997 Thunderbird/Cougar 7/cover 

Honda 1996- 1997 Accord 6/cover behind ashtray 

Honda 1997-1998 AcuraCL 7/open underpassgr. dash 

Honda 1999 Acura CL 8/open bove shifter 

Honda 1999-2000 Acura AL 8/cover In front of shifter behind ashtray 

Honda 1996-1998 AcuraTL · 8/cover behind ashtray . 

Honda 1999-2000 Acura TL 6/cover below radio next to seat heater control 

Honda 1997-2000 CR·V 7/open under P8SSOr. dash 

Honda 1996-2000 DelSol/Hvbrid 7/open under passgr. dash 

Honda 1996-1999 lntegra 7/open under passgr. dash 

Honda 1997-2000 NSX, S2,000 7/open under passgr. dash 

Honda 1996- 1998 Odyssey 7/cover console under passgr. dash 

Honda 1997- 2000 Prelude 7/cover under passgr. dash 

Honda 1996 Prelude 8/open above shifter 

Honda 1996-1998 Acura AL 7/open passenger side center console front 

Hyundai 1996-1998 Accent 2/open In coin box 

Lexus 1996 ES300 2/cover behind fuse box panel 

Lexus 1996-2000 LS400 2/cover above parking brake 

Lotus 1997- 2000 Esprit 7/open Above Passenger Dash 

Mazda 1998-1999 Mlata 2/cover behind fuse box panel 

Mitsubnhi 1996 Expo 2/open behind fuse box 

Porsche 1996 Ail Vehicles 6/cover driver's side of console 

Rolls· Royce 1996-2000 all 9/cover in glove box 

Rover 1997 Defender 6/cover under parcel tray 

Rover 1996- 2000 Range Rover 7/open under passngr dash 

Subaru 1996-2000 LeQacy 2/cover behind plastic hinged cover 

Subaru 1996-1997 svx 1/cover right side of steering column 

Toyota 1996 Avalon 2/cover behind fuse box panel 

Toyota 1996 Camry 2/cover behind coin box 

Tovota 2000 New Hvbrid 7/open 

Toyota 1996 - 1997 Prevla .(2/4 WO) 6/cover top lnstrumt panel 

Toyota 1996-1998 Tercel 2/cover behind fuse box panel 

Volvo 1997-1998 850 8/cover in front of shifter under coin tray 

VOLVO 1998- 1999 all vehicles except Sao 9/cover hand brake area 

Volvo 2000 C/SN70 8/cover 

Volvo 2000 SN40 6/cover 

vw 1996-1998 Cabrio, GoH, Jetta 7/cover right side of ashtray 

vw 1996-1999 Eurovan 4/cover on dash behind wiper lever 

vw 1999 Golf,Jetta 7/cover 

vw 1996 - 1997 Passat 4/cover on dash behind wiper lever 

Page 1 
APPENDIXD 

Manufacturers Known to Have OBD Readiness Issues 

1996 Chrysler vehicles - Vehicles may clear readiness at key-off. Vehicles should be tested 

normally. If vehicles are found to be "Not Ready," they should be referred to a qualified 

service provider so the OBD software can be updated. 

1996 - 1998 Mitsubishi vehicles -These vehicles may have a high degree of"Not Ready" for 

catalyst monitor due to a "trip based" design. Mitsubishi has provided driving cycles in its 

service information to allow monitors to operate. These vehicles should be scanned for MIL 

illumination without regard to readiness status. 

1996 Nissan vehicles and 1997 Nissan 2.0 liter 200SX - These vehicles may have a high 

degree of "Not Ready"for catalyst and evaporative monitors due to a "trip based" design. 

Nissan has provided driving cycles in its service information to allow monitors to operate. 

These vehicles should be treated as other non-problematic vehicles. Nissan Technical 

Service Bulletin #NTB98-018, February 18, 1998. 

1996-98 Saab vehicles - These vehicles may have a high degree of "Not Ready"for catalyst 

and evaporative monitors due to a "trip based" design. Saab has provided driving cycles in 

its service information to allow monitors to operate. These vehicles should be treated as 

other non-problematic vehicles. Saab Technical Service Bulletin not yet available. 

1996 Subaru vehicles - Vehicles will clear readiness at key-off. There is no reprogramming 

available for this line of vehicles. These vehicles should be scanned for MIL illumination 

without regard to readiness status. Subaru Technical Service Bulletin #11 -49-97R (see 

Appendix F of this guidance). 

1997 Toyota Tercel and Paseo - Vehicles will never clear the evaporative monitor to 

"Ready." At this time no fix is available. Vehicles should be scanned using remaining 

readiness monitors as described for non-problematic vehicles. 

1996 Volvo 850 Turbo - Vehicles will clear readiness at key-off. There is no reprogramming 

available for this line of vehicles. These vehicles should be scanned for MIL illumination 

without regard to readiness status. Volvo Technical Service Bulletin #SB 2-23-0056. 

1996-98 Volvo vehicles (excluding 850 Turbo) - These vehicles may have a high degree of 

"Not Ready"for catalyst and evaporative monitors due to a "trip based" design. Volvo has 

provided driving cycles in its service information to allow monitors to operate. These 

vehicles should be treated as other non-problematic vehicles. Volvo Technical Service 

Bulletin #SB 2-23-0056. 
APPENDIXE 

OBD-1/M Test Procedure Flow Chart 
OBD l/M RECOMMENDED TEST PROCEDURE FOR 1996 MODEL YEAR AND NEWER VEHICLES . 

Vehicle fails 

test 

(Note #1) 

Start OBD 

l/M Test 

Procedure 

Center for Automotive Science and Technology 

Weber State University 

No 

Turn ignition 

switch to the 

"Off" position 

for at least 12 

seconds 

Locate DLC, 

connect test 

equipment 

Note MIL (MIL 

Illumination maybe 

brief) while turning 

ignition switch to 

the "Run" position 

with ~ngine "Off'' 

Start and leave 

engine running and 

establish 

communication 

with on-board 

system 

A 
OBD l/M RECOMMENDED TEST PROCEDURE FOR 1996 MODEL YEAR AND NEWER VEHICLES (Cont) 

Connect 

functional 

approved test 

equipment 

Vehicle fails 

test 

(Note #2) 

Vehicle fails 

test 

(Note #4) 

B 

Center for Automotive Science and Technology 

Weber State University 

No 

End 

Yes 

Yes 

No 

Yes 

Yes 

No 

Vehicle fails 

test 

(Note #3) 

B 
OBD l/M RECOMMENDED TEST PROCEDURE FOR 1996 MODEL YEAR AND NEWER VEHICLES 

Retrieve DTCs using 

Mode $03 request. 

Provide DTC's with 

description on a 

Vehicle Inspection 

Report for repair and 

fleet failure 

characterization 

Vehicle 

fails OBD 

l/M test 

End 

View Readiness Code 

status 

(Data collection only) 

Center for Automotive Science and Technology 

Weber State University 

B 

No 

Vehicle is not ready 

for l/M·evaluation. 

Vehicle needs to be 

operated in such a 

way as to exercise all 

monitors. 

(Note #5) 

c 

Yes 

Vehicle 

passes 

080 l/M 

test 

(Cont) 
Notes on flow chart: 

Note 1: The purpose of this step is to verify the On-Board Diagnostic (OBD) system has control of 

the Malfunction lnc:licator Light (MIL) and the MIL is functional. Operation of the MIL varies 

between vehicle manufacturers. Key On Engine Off (KOEO) typically results in the MIL on 

steady, however, there are systems which will illuminate the MIL only briefly during KOEO. 

In either situation MIL presence and illumination capability has been established. 

If the vehicle fails the l/M test at this point, the vehicle inspection report should indicate the 

MIL problem should be repaired and also include information gathered during the 

remaining 1/M test steps. 

Note 2: It is important for the l/M testing personnel to verify proper diagnostic equipment operation 

before failing the vehicle. If the diagnostic equipment is functional then the vehicle's 

communication problem must be resolved. 

Without communication between the OBD system and the test equipment the 1/M test must 

be ended and the problem resolved before further interrogation of the vehicle can be 

performed. This step includes identification of Data Link Connector (DLC) tampering, 

serial data circuit problems and any other condition that would prevent the 08D system 

from communicating with the test equipment. · 

Note 3: l/M test failure is a result of MIL illumination even though the OBD system has not 

commanded the MIL on, or has stored any Diagnostic Trouble Codes (DTCs); e.g., a serial 

data line failure between the OBD computer and the Instrument Panel. 

Note 4: l/M test failure is a result of both the actual and commanded state of the MIL. DTCs should 

be stored since the MIL is commanded on. A vehicle should not fail an l/M test when 

DTCs are stored but there is no MIL on; e.g., the OTC was stored by a loose gas cap which 

was subsequently tightened. 

Note 5: Readiness Code status must be identified at this stage in the l/M test to determine whether 

or not all emission control systems have been tested by the 08D system. If any one (or 

more) Readiness Code(s) are not set ("ready'') the 08D system has not yet completed 

testing of the system(s) and failures may be present but not yet identified. It is important to 

understand that the vehicle does not fail the l/M test at this point; no emission related 

faults have been identified. The current state of the vehicle's emission control system is 

undetermined. 

The emission control systems and related components are tested under specific vehicle 

operating conditions. Therefore, to set the Readiness Codes the vehicle must be operated 

within these specific conditions (commonly referred to as "enable criteria") for the OBD 

system tests to be performed. Once testing of an emission control system is complete, the 

related Readiness Code will be set ("ready"). When all Readiness Codes are set, the 

vehicle is ready for further l/M testing. 

It will be at the states discretion whether to recommend the customer drive the vehicle to 

set the Readin.ess Codes or to take the vehicle for service. The state may also choose to 

use a dynamometer drive cycle. 
Note 6: EPA has revised the current readiness code requirement to allow states to complete the 

testing process on model year 1996 thru 2000 vehicles with two or fewer unset readiness 

codes; for model year 2001 and newer vehicles, the testing process could still be complete 

provided there is no more than one unset readiness code. It is important to understand 

that the vehicle does not fail the l/M test because an unset readiness code is not itself an 

indication of an emission problem with the vehicle.