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Q&As: On-Board Diagnostic Systems

What is OBD?

On-Board Diagnostics (OBD) is a term used to describe a vehicle's computer system that controls the emission control, along with many other features. This system includes self-diagnostic and reporting functions. Most 1996 and newer vehicles less than 14,000 pounds gross vehicle weight rating (GVWR) (e.g., passenger cars, pickup trucks, sport utility vehicles) are equipped with second generation OBD systems also known as OBD II.

What are the benefits of OBD II?

OBD II systems monitor the vehicle's emission control system performance and notify the driver when defects that cause an increase in air pollution are identified. These systems have the ability to alert the driver of a problem using a Check Engine light (also known as a Malfunction Indicator Light, or MIL) located on the vehicle's dashboard. OBD II also stores important information about a detected malfunction that will help a repair technician identify and correct the problem. Early diagnosis and timely repair benefit air quality, may improve fuel economy, and can prevent major repair bills.

Check Engine

Should I continue to drive my vehicle if the Check Engine light stays on or is blinking?

If the Check Engine light comes on, take your vehicle to a licensed repair station as soon as you can to have the problem diagnosed. A blinking or flashing light indicates a malfunction that should be addressed immediately to avoid serious damage to the engine or emissions control system. Check your owner's manual for repairs that may be covered under your vehicle manufacturer's emissions warranty.

Will my car receive an OBD II test?

Model-year 1996 and newer vehicles equipped with OBD II require an OBD check as part of their official Smog Check inspection, which includes a visual, functional, and a tailpipe inspection. Vehicle model-years 2000 and newer only receive an OBD II test (BAR-OIS) and a visual inspection. There is no tailpipe inspection of these 2000 and newer vehicles. For 1996-1999 model-year vehicles, the Smog Check will also include a tailpipe test and a visual inspection.

How can I help my vehicle pass the OBD II test?

Performing regular and proper vehicle maintenance according to your owner's manual and not tampering with the emissions control equipment are keys to passing Smog Check.

How much does the BAR-OIS test cost?

BAR does not regulate the price of inspections, so the cost of a Smog Check varies from station to station. Consumers should shop around for the best price.

What are some of the ways a vehicle can fail the new BAR-OIS test?

The following list includes possible reasons why a vehicle may not pass the BAR-OIS inspection:

  • The vehicle's check engine light (malfunction indicator light - MIL) is illuminated or flashing. The vehicle turns this lamp on when the OBD system detects a fault found when running its self-diagnostics. Diagnostic Trouble Codes (DTCs) typically accompany an illuminated check engine light.
  • The check engine light fails to illuminate during the bulb check. When the key is turned on and the engine is off, the bulb should turn on for a few seconds to prove that it is working.
  • A number of monitors are "not ready." This means the vehicle has not had the opportunity to complete one or more of the required self-diagnostic checks of the vehicle's OBD system. The number of allowed unready monitors varies by model-year and fuel type.
  • The OBD data retrieved from the vehicle does not match what is expected for that vehicle. Plugging into an incorrect vehicle to perform the OBD inspection, or a tampered OBD system, can cause this failure.
  • The vehicle fails to communicate with the OBD inspection equipment. This failure is typically caused by a vehicle design defect, wiring, or damaged OBD under dash connector.

If your vehicle fails its Smog Check, ask the inspector to provide an explanation of the reason for the failure and seek a repair technician to correct the problem and get your vehicle's Smog Check certification.

What if my vehicle fails the BAR-OIS test?

If a vehicle fails Smog Check, you should schedule an appointment for a diagnosis at a station licensed to perform Smog Check repairs. This is important because some stations ("Test-Only") are licensed only to perform the Smog Check test itself. Visit BAR's station locator page to locate a licensed Smog Check Test and Repair station in your area.

If there is a dispute about the results of your Smog Check inspection, call the Referee call center at 1 (800) 622-7733 to make an appointment for an inspection.

How can I learn more about OBD?

Check your vehicle owner's manual or visit the Air Resources Board (ARB) website at for more information.