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Your Check Engine Light is On... What Does This Mean?

Beginning in 1996, all cars and light trucks sold in the United States and Canada include a common diagnostic system, On Board Diagnostic ll (referred to as OBD II). OBD II is a diagnostic system residing within the vehicle that monitors basic functions of the fuel injection system, ignition system, and emission control system.

When the OBD II system detects a fault in one of the components of these systems — or just simply detects the vehicle is not running efficiently — it does three things:

  1. The OBD II system records a diagnostic trouble code in its memory, so the repair technician can access these codes to facilitate a repair.

  2. The system turns on the Check Engine Light. The light will stay on until the technician turns off the light when he repairs the problem, or the system may turn off the light when no faults are detected for several runs.

  3. If the fault is serious enough, the OBD II system will put the affected system into default mode, to try to minimize any potential damage, and keep the vehicle running long enough to get you off of the road. In this case, the vehicle may run with reduced power. The Check Engine Light may start flashing to note that damage is possible.


If the Check Engine Light comes on, an assessment must be made to determine if you should still drive the vehicle or have it towed to the repair facility. If the light is not flashing, and the vehicle is running well, it may be driven.

Never drive a vehicle with the temperature gauge reading hot or with any indication of oil problems. Do not drive if the engine is jerking or vibrating noticeably.

If the problem is serious enough, unburned fuel may go into the exhaust system, and when this fuel is burned, it will ruin the catalytic converter and may cause a fire under the car.

In Shop Diagnostic Work

There is an equipment and diagnostic fee for the technician to read and analyze the OBD II system. The actual repair is a separate charge.

The technician will read and interpret the codes and then take action.

If, in the technician's judgment, the codes indicate a singular event such as a load of bad fuel or the gas cap was left off, he may just choose to reset the light, rather than initiate unnecessary repairs.

If the codes are suggestive of a specific problem, the technician will suggest a repair or further diagnostic work.

On occasion there may be so many codes in the system from past occurrences or some unusual event that the codes may not make any sense. In this case, the technician may suggest resetting the light and erasing the codes to see if any of them repeat after driving for a couple of days. In such a case, any charges to the vehicle owner are for diagnostic work and not for any repair work.

If your Check Engine Light is on and you have any questions, please contact us.