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What Every Motorist Should Know About vehicle Emissions Testing

by Larry Carley copyright AA1Car.com

Emissions testing has been a controversial subject ever since its inception. Though most opinion polls show widespread public support for clean air in general, few motorists show any enthusiasm for emissions testing when it involves their own vehicles. And most are reluctant to spend money on emission-related maintenance or repairs.

As long as most vehicles pass an emission test, most people will go along with a program, pay a reasonable test fee and tolerate waiting in line 20 to 30 minutes once every year or two to have their vehicle tested. But when their vehicle fails an emissions test, their attitude often becomes angry and resentful. An emissions failure creates stress and anxiety because of what comes next.

A failure means finding a shop with technicians who are competent enough to do emission repairs, making a service appointment, being without a vehicle for half a day or more, having to spend up to several hundred dollars or more on emission repairs they may not even believe are really necessary, and then taking the vehicle back to the inspection station for retesting. And if the vehicle fails the retest? They feel even more frustration and anger as they bounce back and forth between the repair facility and test station. Consequently, there has been a lot of public backlash against emission test programs that are too stringent or fail too many vehicles.

A growing number of people today are questioning the value of emissions testing, and wonder if it is making any significant difference in reducing air pollution. U.S. Environmental Protection Agency (EPA) statistics show that air quality is improving in most areas of the country, but the data fails to show a direct link between the reductions in pollution from mobile sources (vehicles) and emission testing. Some areas that have no inspection/maintenance (I/M) programs have shown just as much improvement in air quality as areas that do emissions testing.

Most of the reduction in emissions from mobile sources is being attributed to changes in the vehicle population. As older vehicles are replaced by newer, cleaner running vehicles, the amount of pollution from mobile sources has gone down and will continue to decline as time goes on.

Not only are new vehicles much cleaner, they also stay cleaner for a longer period of time. Even so, older vehicles (10 years old or more) continue to be a significant part of the vehicle population and represent a major source of pollution. Consequently, periodic emission testing is seen as a necessary means of policing these older vehicles.


According to the National Center of Vehicle Emissions Control & Safety (NCVECS), Colorado State University, various issues confront vehicle emissions testing today:

Little political or public support for emissions testing.

Lax standards and poor enforcement of existing I/M programs.

Lack of credibility that existing I/M programs are having an impact on air quality.

Reluctance to implement effective enhanced I/M programs in non-attainment areas.

Political pressure on the EPA to be more "flexible" in accepting various emission testing alternatives.

Rising costs of administering and conducting enhanced emissions testing.

Less need to inspect newer vehicles.

Adding OBD II checks into existing I/M programs, or substituting OBD II checks for tailpipe tests on 1996 and newer vehicles.

Technician training and competency.

History of Emissions Testing Legislation

In 1990, Congress amended the Clean Air Act. The revisions required areas that did not meet national ambient air quality standards (NAAQS) to implement either basic or "enhanced" vehicle I/M emissions testing programs, depending upon the severity of the area's air quality problem. The act also required that metro areas with populations of more than 100,000 implement enhanced I/M emissions testing regardless of their air quality designation. EPA, in turn, was required to develop standards and procedures for emissions testing.

On November 5, 1992, EPA issued its original rule establishing minimum performance and administrative requirements for states developing air quality implementation plans. The EPA said areas that needed enhanced emissions testing would have to use their new I/M 240 test procedure. I/M 240 was controversial for several reasons. One was that it specified centralized testing. The EPA said the use of "test only" facilities administered by an independent contractor would eliminate any conflicts of interest (fraud) in shops that both test and repair vehicles. California garage owners balked at the requirement, and eventually forced the EPA to accept a hybrid decentralized program in their state.

The I/M 240 requirement also specified loaded mode testing for measuring transient emissions on a special dynamometer, as well as checking NOx emissions and doing an evaporative system purge and pressure test. The I/M 240 test was based on procedures the EPA had already developed for certifying new vehicle emission compliance. This, in turn, required a lot of expensive equipment as well as the use of a trained operator to follow a prescribed drive cycle while the vehicle was on a dyno.

In 1995, however, the National Highway System Designation Act was passed. The act included provisions that specifically barred the EPA from mandating I/M 240 exclusively for enhanced emissions testing. So the EPA was forced to adopt a more flexible posture toward alternative I/M test programs. States are still required to meet air quality standards, but now have a much wider range of options for meeting those standards. These include scrapping programs for taking older vehicles off the road, the use of onboard diagnostic system (OBD) testing for verifying emissions performance, the use of decentralized I/M programs, roadside testing and various enhanced test procedures such as acceleration mode testing (ASM) and others that have been developed as alternatives to I/M 240.

For example, it is currently possible for some areas to design emissions testing programs that meet the required enhanced I/M performance standard without any tailpipe testing at all, using a combination of alternative evaporative system pressure testing methods, onboard diagnostic system checks, and visual anti-tampering inspections.

Many states are now using a process called "clean screening" to simplify emissions testing. The goal here is not to identify high polluting vehicles for repairs, but to identify especially clean vehicles, which can be exempted from routine testing. Some states now exempt new cars from emissions testing until they are two or more years old, and then only require testing every two years thereafter.


I/M 240 has pretty much disappeared due to its cost and complexity. Most states are now using a simple Plug-in OBD II Emissions Test. The onboard diagnostic system in late model vehicles does an excellent job of monitoring emissions compliance. It will set diagnostic trouble codes (DTCs) and turn on the Check Engine Light if a problem occurs that may cause emissions to exceed federal limits by a specified amount (typically 1.5X).

For the latest information on current state emissions testing programs, see OBD Program Status or Vehicle Emissions I/M Programs.

States that implemented some type of enhanced emissions testing (I/M 240 or similar tests) to measure tailpipe emissions during transient operating modes on a dyno, or acceleration simulation mode (ASM) tests, have mostly found that a plug-in OBD II check works just as well while eliminating the risks of placing a motorist's car on a dyno and running it at highway speeds.

As the vehicle population continues to get cleaner, and new vehicles meet even low emission vehicle (LEV) and ultra-low emission vehicle (ULEV) standards, the cost of vehicle emissions testing programs will likely come under close scrutiny by legislators. Some areas may opt to phase out their annual or biennial emissions inspections and replace them with roadside sniffers and profiling to zero in on problem vehicles.

California is looking at standards for what may eventually become OBD III technology. The next generation OBD system could use wireless cell phone technology in conjunction with the onboard diagnostic system to report a vehicle's emission status to the state. As long as the vehicle's emissions are in compliance, there would be no need to bring it in for a test (saving motorists and the state time and money). But if the vehicle developed a problem, it would then have to be repaired and/or tested to bring it back into compliance.

Emissions Test Standards

As we move forward, the amount of pollutants allowed in the exhaust continues to be reduced. That includes carbon monoxide (CO), unburned hydrocarbons (HC), formaldehyde (HCHO), non-methane organic gases (NMOG), oxides of nitrogen (NOx) and particulate matter (PM). Here are some current and future federal and California emission standards:

Federal emission standards Tier II

Federal emission standards for oxides of nitrogen NOX

California emission standards LEV II

More Emissions Articles:

Plug-in OBD II Emissions Testing

Evolution of I/M 240

Understanding OBD II Driveability & Emissions Problems

Fixing Emission Failures

OBD Monitor Not Ready

All About Onboard Diagnostics II (OBD II)

Basic Emission Control Systems Overview

Exhaust Emissions Diagnosis

Troubleshooting a P0420 Catalyst Code

Catalytic Converters

Exhaust Gas Recirculation (EGR)

PCV System

EVAP Evaporative Emission Control System

Driveability Diagnosis: Misfires

Finding & Fixing Vacuum Leaks

Understanding Oxygen (O2) Sensors

Wide Ratio Air Fuel (WRAF) Sensors

Sensing Emission Problems (O2 Sensors)

Sensing Emission Problems (O2 Sensors)

Decoding Onboard Diagnostics


Most Common Trouble Codes (and what causes them)

All About Onboard Diagnostics II (OBD II)

Understanding OBD II Driveability & Emissions Problems

OBD II Engine Diagnostics

The Environmental Challenge Now Is CO2

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