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On-Board Diagnostic Testing Program

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Barry R. Stephens, Director
William R. Snodgrass Tennessee Tower
312 Rosa L. Parks Avenue, 15th Floor
Nashville, TN 37243
(615) 532-0554
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Vehicle Testing Questions?

For Questions Including What To Do If You Fail Your Emissions Test:
Call the toll-free TDEC Vehicle Testing Information Hotline: 1-866-329-9632

For Questions Regarding Middle Tennessee and Hamilton County Testing Station Locations andHours of Operation:
Call the Middle TN Envirotest
Information Hotline: 615-399-8995
Or Call the toll-free Envirotest
Information Hotline: 1-877-477-0800

Information and Testing Requirements for 1996 and Newer Model Vehicles

1996 and newer model vehicles undergo an On-Board Diagnostics test, and a gas cap leak check instead of the traditional tailpipe and tampering test.

On-Board Diagnostics tests significantly enhance the overall emissions reductions achievable by the vehicle emissions testing program.

What is On-Board Diagnostics-II (OBD II)?

On-Board Diagnostics II is the second generation of on-board self-diagnostic equipment on vehicles. This system is designed to provide the driver with an early warning of potentially high emissions, and stores important information about detected malfunctions so that a repair technician can accurately find and fix the problem.

How is the driver notified?

A light on the dash, called a Malfunction Indicator Lamp (MIL), will come on if an emissions related problem is detected.

Your vehicle's OBD II system will turn the MIL light "ON" when it finds a fault that may cause high engine or evaporative emissions. In some instances, this may mean that immediate service is needed to prevent expensive damage and ultimately, high emissions. (Check the owner's manual.)

How is an OBD II inspection done?

It involves two simple steps:

With the key on and engine off, the MIL light is checked to verify that it works.

A scan tool is connected to the vehicles on-board computer system which checks the systems status.

Why is OBD II needed?

Even though newer vehicles are the cleanest manufactured, rapid growth is multiplying the number of cars on the road today, and the ever-increasing miles they travel each day. Their emissions make them a major contributor to pollution. While new vehicles start out with very low emissions, improper maintenance or faulty components can cause the vehicle emission levels to sharply increase. Studies estimate that approximately 50% of the total emissions from late-model vehicles are the result of emission-related malfunctions. OBD II works to ensure that the vehicles remain as clean as possible over their entire life.

Does my car have OBD II?

All 1996 and newer gasoline and alternate fuel passenger cars and trucks are required to have OBD II systems. All 1997 and newer diesel fueled passenger cars and trucks are also required to meet the OBD requirements. Additionally, a small number of 1994 and 1995 model year gasoline vehicles were equipped with OBD II systems.To verify if your vehicle is equipped with OBD II, you can look for the words "OBD II" on the emission control information label attached to the underside of the vehicle hood.

Do other states require OBD II?

Yes. The United States Environmental Protection Agency (EPA) requires all 1996 and newer vehicles sold in any state to meet the EPA OBD requirements.

What should I do if the MIL warning light comes on?

Most manufacturers advise having the vehicle serviced as soon as conveniently possible. Since there are many different problems that can cause the light to illuminate, it is hard to generalize how severe a problem may be. However, often the problem will have a noticeable effect on fuel economy, performance, or driveability of your vehicle, and extended driving without fixing the problem could possibly even damage other components.There are certain malfunctions that can cause the warning light to blink. This indicates that a malfunction is currently occurring which could be damaging your catalytic converter. Because replacement of the catalyst can be expensive, many manufacturers recommend having the vehicle serviced as soon as possible.

Does the warning light only mean the emissions controls on my car aren't working?

While all malfunctions that cause the light to illuminate do affect emissions, many also can affect fuel economy and several can cause driveability problems or a decrease in overall performance. Manufacturers generally optimize their vehicles for performance, fuel economy, and emissions. As such, virtually any malfunctioning component can result in the vehicle running in a condition that is less than optimal.

Do I have to go to the dealer to get my OBD II car fixed?

No. Properly trained and equipped independent shops are capable of utilizing the diagnostic information from the OBD II system and can make repairs just like dealers. In fact, several of the provisions incorporated in the OBD II regulation are intended to make it easier for shops to diagnose and repair vehicles accurately and in a cost-effective manner. Note: Some repairs may under warranty. Check with the manufacturer or your owner's manual for specific details and coverage.

Will 1996 and newer vehicles still undergo the 3 Point tampering check?

No, the OBD II System checks the emissions system, so the only additional check that will be performed is a gas cap leak check.

How is a gas cap leak check done?

The gas cap is removed, and a pressure test is performed on the cap to ensure there is an adequate seal to prevent gasoline vapors from escaping into the atmosphere. A pass/fail determination is made, and upon completion of the test, the cap put back in place.

SOME IMPORTANT FACTS

What does it mean if your vehicle was unable to be tested because the ODB System was "Not Ready"?

Readiness codes are indicators used to find out if emission components are working correctly. If all monitors are set to ready, the emission components have been tested. If the monitors are not set to ready, this does not mean that the vehicle has not been repaired, it simply means the computer system has not yet recognized the repair. Monitors not set to "ready" can prevent you from completing an emissions test.

How do I get my vehicle's OBD System "Ready"?

A vehicle must be driven to set monitors to "ready". These operating conditions include a mix of highway driving, stop and go city driving, and at least one overnight off period (known as drive cycle). Some owner's manuals explain drive cycles or procedures needed to reset monitors. The vehicle manufacturer or a qualified service technician is the best source for this information. Each manufacturer has specific criteria that must be met for the readiness codes to be set.

If you have any questions about vehicle emissions testing, you can call:

(615) 532-8378 or Toll-Free 1-866-329-9632