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On-Board Diagnostic System FAQ

The 1990 Clean Air Act required a generic on-board diagnostic computer system to be standard on all vehicles sold in the United States beginning with the 1996 model year and all light duty (vehicles with a manufacturer's gross vehicle weight rating of 8,500 pounds or less) diesel fueled vehicles beginning with the 1997 model year. This requirement replaced computer systems on cars that varied from one vehicle manufacturer to another. The new computer system was named “On-Board Diagnostic, Generation II” or “OBDII.”

The CAA also required that vehicle emissions inspection programs around the country begin inspection of the OBDII system. Effective July 1, 2005, DEQ implemented the new OBDII inspection procedure which became the official inspection process for most 1996 and newer motor vehicles.

The following information is presented to provide an understanding of OBDII testing. For additional information, please contact Air Check Virginia at (703) 583-3900 or toll-free in Virginia at 1-800-275-3844.

On-Board Diagnostics (OBDII) Frequently Asked Questions

What is OBDII? How does it work?

Onboard diagnostics systems were developed to help technicians diagnose and service vehicles' computerized engine management systems, and to inform the motorist when a potential problem exists regarding the emissions control systems. Beginning with the 1996 model year, all light duty cars and trucks are required to monitor specific systems using generic criteria for the evaluation and reporting of system status, and for indicating problems to the motorist and/or technician. This new system is known as "OBDII."

Malfunction Indicator Lamp

The engine management systems are monitored for conditions that could cause tailpipe emissions to increase. When a potential problem is detected, a warning light called a "malfunction indicator light" (MIL) on the dashboard is illuminated to alert the driver of a problem. This light is usually identified by the words "Check Engine," "Service Engine Soon," or the word "Check" along with an engine symbol. Severe emissions-related engine malfunctions may cause the malfunction indicator light to blink or flash. This indicates a need to reduce speed and to seek immediate service.

When the OBDII system detects a problem, a "Diagnostic Trouble Code" (DTC) is stored in the computer's memory. This DTC can be retrieved by using available code-reading devices, such as those used by repair technicians to help diagnose problems with the vehicle. 

The OBDII system can detect problems well before the driver may notice any loss of performance and can help motorists avoid costly repairs to their vehicle. A vehicle identified by the OBDII system as having a problem may be operating inefficiently which could result in poor fuel economy and performance and shorten the life of the engine or other components such as the catalytic converter.


What does this have to do with air pollution?

Motor vehicles are the largest single source of toxic and smog-forming air pollutants in Northern Virginia and much of the rest of the country. Modern vehicles are getting cleaner due to technological advances in emissions control strategies, but emissions are reduced the most when all of the systems are working properly. When an engine is not running efficiently, performance is reduced, fuel is wasted, and exhaust emissions can skyrocket. OBDII can detect problems and identify the system(s) affected and often help lead trained technicians directly to the source of the problem. Many of these problems are not visually noticeable because they are electrical or chemical in nature. By detecting component deterioration and/or failure and alerting the motorist of the need for potential repair, OBDII enables the motorist to bring the vehicle in for proper service before more serious and expensive problems develop.


What happens during an OBDII emissions inspection?

The OBDII inspection provides a more comprehensive picture of vehicle emissions than the traditional tailpipe emissions test. This is because the OBDII system tests the vehicle while it is actually being operated under a variety of day to day driving conditions, whereas the traditional tailpipe test measures emissions only at a particular moment in time. The emissions inspection procedure for most 1996 and newer OBDII equipped vehicles is as follows:

  • The inspector will perform a free preliminary inspection of your vehicle. If problems are found, the vehicle will be rejected from testing until they are corrected.

Note:  The preliminary inspection does include an examination for tire condition on all gasoline powered vehicles.  This is because some vehicles will be required to have a tailpipe emissions test performed in addition to the OBDII test. Diesel powered vehicles will not be rejected from testing due to tire condition or a missing fuel cap. Diesel vehicles with a manufacturer’s designated gross vehicle weight rating of 8,501 pounds or more may receive a Rejection from Testing Form from the inspection station with instructions to contact the Department of Environmental Quality.

  • If the vehicle is safe to test, the inspector will enter the vehicle information into the emissions analyzer system.
  • The inspector will determine whether or not the malfunction indicator lamp is operable, and enter this information into the analyzer.
  • The inspector will then attach a data link cable to the vehicle's onboard computer diagnostic link connector. The emissions analyzer will then evaluate the status of the OBDII system’s monitors to check whether or not the vehicle is ready for an OBDII inspection.

    OBDII monitors are a very important aspect of the inspection process. Monitors are programs in the vehicle's on-board computer that run checks on components and systems of the vehicle for conditions that could cause excessive emissions. To learn more about OBDII monitors, click here.

    If more than two of the monitored systems are found to be "not ready" for 1996 to 2000 model year vehicles, or more than one monitored system is found to be "not ready" for 2001 and later model year vehicles, the vehicle is rejected from testing at no charge and the vehicle must be operated until the monitors have had a chance to run and the system is ready to test.

Note:  If the vehicle failed the OBDII inspection for a catalytic converter related diagnostic trouble code, then the catalytic converter monitor must have run and be in a ready state in order for the reinspection to be performed.

  • Once the vehicle has enough of its monitors and is determined to be "ready," the emissions inspection will proceed.  If no Diagnostic Trouble Codes (DTCs) are found, the vehicle will pass this part of the OBDII inspection process.  If DTCs are found, the analyzer will download up to five of these DTCs that are stored in the vehicle's onboard computer. These DTCs will be printed on the Vehicle Emissions Inspection Report along with a brief, generic explanation of the code if available.

Note: The analyzer will only download and record up to five diagnostic trouble codes. Further diagnostic work by a qualified technician should be performed in order to obtain any additional diagnostic trouble codes (generic or manufacturer specific) that might be stored in the vehicle’s OBDII computer.

  • For gasoline powered vehicles, the OBDII inspection will also include a visual inspection for the presence of the catalytic converter and a visible smoke inspection. Diesel powered vehicles will only get the OBDII inspection.
  • The fee will remain the same for the emissions inspection regardless of which procedure (OBDII or tailpipe emissions test) is performed on the vehicle.


What should I do if my vehicle fails the OBD II inspection?

Vehicles that fail the initial inspection are eligible for one free retest at the original station within 14 calendar days of the initial inspection.

Follow this link to learn more about emissions inspection failures, the repair process, and emissions inspection waiver information.


Can anyone service the OBDII system?

Only qualified, trained service technicians equipped with the latest diagnostic and repair equipment should be employed to conduct OBDII related service on your vehicle. Vehicle owners should ask their service provider if their technicians have had the proper training and have access to the necessary equipment and reference material to properly service OBDII systems.

Certified Emissions Repair Facilities are required to have diagnostic equipment capable of communicating with OBDII systems to download Diagnostic Trouble Codes.  These facilities are also required to employ at least one Certified Emissions Repair Technician who has the knowledge to perform diagnostic and repair work on OBDII vehicles.


How does OBDII help the repair shop fix the vehicle?

When the vehicle is presented for repair, the technician will retrieve the stored diagnostic trouble codes from the vehicle's OBDII computer using a "scan tool." The information obtained will assist the technician in isolating the particular system(s) having the problem.

It is important to note that, while the OBDII system may identify specific systems and certain operating condition information, this is often only part of the job. It will usually still take diagnostic skill, knowledge and additional research to determine the cause. It could be that the sensor itself is at fault, or a loose connection, or the vehicle's computer is causing the problem, or that the component that the sensor monitors is defective. The only way to really make an accurate diagnosis to have the proper equipment, reference material and training.

Some facilities build their diagnostic time into the overall labor charge, and some charge diagnostic time separately. It is important to understand that diagnosing any emissions problem can take time, and it is understandable for a business to charge for this time.


Why am I charged diagnostic time for a system that diagnoses itself?

OBDII only provides a hint or a clue of the problem. Diagnosis time will generally be needed to isolate the true cause of your vehicle's problem and identify the proper repair. There will usually be a charge for this labor but this can save you money in the long run, as it can minimize unnecessary repairs.


How can the light be turned off?

After fixing the problem, the service technician may be able to reset the malfunction indicator light. When the malfunction indicator light is reset, the vehicle will usually have to be operated normally for a day or two to allow the OBDII system to complete its pre-programmed tests, or monitors, before the vehicle can be re-inspected. Click here to read more about OBDII monitors

There are also situations under which the OBDII system will automatically turn off the malfunction indicator light if the conditions that caused the problem are no longer present. This may be the case after repairs to certain components that require special driving conditions for the system to test itself. If the OBDII system evaluates a component or system three consecutive times and no longer detects the initial problem, the malfunction indicator light will automatically turn off. For example, if the gas cap is not properly tightened after refueling, the OBDII system may detect vapor leakage and illuminate the malfunction indicator light. If the gas cap is then tightened, the OBDII system will recognize this and the light will likely be turned off after a few days of driving.


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Virginia Department of
Environmental Quality
P.O. Box 1105
Richmond, VA 23218
(804) 698-4000

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