United States

Environmental Protection

Agency

Air and Radiation EPA420-R-01-015

June 2001

Performing Onboard

Diagnostic System

Checks as Part of a

Vehicle Inspection and

Maintenance Program

 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 is to facilitate the exchange of

technical information and to inform the public of technical developments which

may form the basis for a final EPA decision, position, or regulatory action.
2

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-I/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-I/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-I/M pass, failure, or rejection. Lastly, the August 6, 1996

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

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

At the time the original OBD-I/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 OBD-equipped vehicles in need of repair proved more difficult and

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

assessment of OBD-I/M effectiveness and implementation issues referenced in this guidance. 
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 I/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 I/M, only one county – Davis – has opted to begin early implementation

of the OBD-I/M check.

3

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

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

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

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

requirements by publishing a final rulemaking (FRM) in the Federal Register on April 5, 20011

. 

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-I/M testing. Among

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

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

clarify states’ options regarding the integration of OBD-I/M checks into existing I/M networks;

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

OBD-I/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 OBD-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-I/M testing in an I/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 I/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 (FACA). 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, California2

, 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.
4

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

the performance of OBD testing in the I/M 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 OBD-I/M 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 OBD-I/M 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 I/M implementation issues which are

common to all test types and for which there are no unique, OBD-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 I/M implementation issues by consulting the I/M rule (as amended) and EPA’s

subsequent I/M 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 OBD system has been an option on certain vehicle models

since the early 1980's, standardized OBD systems (also known as OBD 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 I/M purposes, EPA does not require that pre-1996 MY

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

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

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

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

administratively very difficult. Additionally, EPA’s MOBILE6 emission factor model will not

provide emission reduction credit for the performance of OBD-I/M checks on pre-1996 MY

vehicles.

EPA also does not recommend that vehicles older than MY 1994 be subjected to OBDbased I/M testing, even if it is determined that the vehicle is equipped with an OBD computer,
4 40 CFR 85.2222 “On-board diagnostic test procedures” requires that the scan tool used for the OBD-I/M inspection

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

Recommended Practice J1979.

5 While it is possible to perform the electronic scan portion of the OBD-I/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

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

recommend performing an OBD-I/M scan on pre-1994 MY OBD-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 I/M programs4

. 

Elements of an OBD-I/M Check

An OBD-I/M 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. 
6 For I/M 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 J1979 MODE 01 PID 01 DATA C and D.

8 Refer to SAE J1979 MODE 01 PID 01 DATA A BIT 7.

9

 EPA’s original OBD-I/M failure criteria were limited to power-train, emission-related DTCs (refer to SAE J1979

MODE 03). In its April 5, 2001 rulemaking, however, EPA simplified the failure-triggering DTC criteria to any DTC 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 of OBD-I/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 I/M failure rates -- especially not in those

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

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

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

6

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 tool10. 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
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-I/M programs, and Weber State

University to develop an online clearinghouse of OBD-related information useful to state I/M 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-I/M 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

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-I/M check on

OBD-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 tampered12 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
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 if the 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

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-I/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
9

I/M-Related DTCs

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

DTCs as being relevant for I/M purposes. If a vehicle was identified through an I/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-I/M check. Furthermore, EPA

has examined data from over 300,000 OBD-I/M checks performed in actual I/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 I/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
10

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

OBD-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 J2012 and J1930. 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., if the 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),
15 As part of a 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-I/M testing (or retesting).

11

“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 I/M program requirements by clearing OBD codes just

prior to OBD-I/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 OBD-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
16 As discussed above, when determining the readiness status of a 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-I/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 MILs 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 MILs, 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-I/M program, which is currently failing

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

that roughly 1.4% of MY1996+ 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

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-I/M data and also

reflects a FACA workgroup recommendation. Since August 1998, Wisconsin’s I/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

OBD-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
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 86.094-17(e)(1): “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 I/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 I/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

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

requirements18. 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-I/M

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 I/M. 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 I/M 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
14

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

retesting after repairs have been performed to correct an initial OBD-I/M 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-
19 In looking at data from the Oregon OBD-I/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-I/M check, the average

number of retests 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

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, OBD-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 I/M

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 OBD-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 turn 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., OBD-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 small19. 

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 --
20 Refer to SAE J1979 - “E/E Diagnostic Test Modes,” Mode $04.

16

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-I/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-I/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 key-
21 During its pilot testing of OBD 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

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 I/M 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-I/M 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. 
18

Implementation Issues for Centralized vs. Decentralized OBD-Based Testing

While EPA recommends that I/M programs integrate OBD-I/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 I/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-I/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-I/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 I/M test types, the OBD-I/M check is vulnerable to

inspector fraud, and program managers need to be on guard to limit the opportunities for this
23 Refer to SAE J1979 MODE 01 PID 00.

24 Refer to SAE J1979 Section 4.2.4 Header Bytes.

19

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 OBD-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 OBD

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 OBD-I/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 OBD-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 OBD

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

for OBD-tested vehicles, EPA recommends that states consider at least modifying waiver
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-I/M test with the

exception that vehicles would not be failed on the basis of the OBD-I/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-I/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

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 I/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 OBD-equipped vehicles

into I/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
21

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 OBD-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 OBD

scan as needing service and/or repair. States should work with their local educational

institutions, OBD 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

OBD-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, OBD-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 OBD (i.e., theory, terminology, legal requirements, etc.)

& The differences between OBD I and OBD II

& The OBD-I/M inspection procedure

& The pass, fail, and rejection criteria for OBD-equipped vehicles

& Readiness, the setting and clearing of codes, and MIL-triggering vs. pending

DTCs

& The link between the OBD-I/M check and the environment, and

& Proper diagnostic procedures and available sources of diagnostic materials

(i.e., manufacturers, hotlines, web sites, etc.). 
APPENDIX A

Glossary of I/M- and OBD-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/M is 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 (DTCs): 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 I/M: A vehicle inspection and maintenance program designed to meet one of three

enhanced I/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 I/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 (OBD): 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 I/M 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 I/M 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. 
APPENDIX B

Data Link Connector Mapping Diagram

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).
APPENDIX C

Vehicles with Hard-to-Find Data Link Connector Locations*, by Make and Model Year

(*Location numbers refer to DLC map in Appendix B)

APPENDIX D

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.
APPENDIX E

OBD-I/M Test Procedure Flow Chart