CHECK ENGINE LIGHT OPERATION DIAGNOSTIC/CAUSES & CORRECTION

The "check engine" light is used on all C-3 systems to indicate faults in certain electrical circuits. Even though the diagnostic system has been on our cars for three (3) years (1980 California), there are still misunderstandings about its operation. The following is a review of previously published information in an attempt to clarify the meaning of a "check engine" light.

If a fault occurs in certain C-3 circuits, the "check engine" light will come "ON" and a trouble code will be stored in the ECM memory. If the fault corrects itself or goes away, the "check engine" light will go out ten (10) seconds after the fault is gone. However, the trouble code will remain in the ECM memory until battery power to the ECM is interrupted. This battery power is supplied, on most cars, through an ECM fuse in the fuse block to ECM Terminal #R.

For most faults to set the "check engine" light, the engine must be operated for a period of time under certain conditions. This is done to prevent setting false codes.

With certain faults (Codes 13, 21, 24, 34, 35, 44 and 45), the "check engine" light will come "ON" and go "OFF" according to driving conditions such as vehicle speed, engine rpm, throttle position, etc. Thus, the light will go "ON" and "OFF" even though the fault remains.

Other faults will turn "ON" the "check engine" light after some time delay and it will remain "ON" until the ignition is turned "OFF" (Codes 14, 15, 32 and 51). In these cases, once the light comes "ON" it will remain "ON" regardless of vehicle operation until the ignition is turned "OFF."

The remaining codes (12, 23, 41, 42, 43, 54 and 55) will turn "ON" the "check engine" light within ten (10) seconds of detecting the fault and will remain "ON" as long as the engine is running. It is not necessary to meet certain time and operating conditions before the light comes on. Thus, the "check engine" light will be on whenever the engine is running as long as the fault is present.

In all cases, if the fault goes away or correct itself, the "check engine" light will go off, but the trouble code will be stored.

One (1) of the first diagnostic steps is to determine whether the condition is an intermittent or a hard fault. This is done during the diagnostic circuit check. The car must be run for two (2) minutes at curb idle. This allows codes that can set at idle to set. Intermittent conditions or codes that require driving the car are covered in the specific code charts.

Another difficult situation is a "check engine" light on with no codes stored. If the light is "ON" steady with the engine running, the cause is likely to be a ground power feed condition or a lamp driver malfunction on 1982 models. Loss of 12V to Terminal "R" may cause this condition on 1981 models. In a few instances, this condition may be due to a faulty ECM or grounded "check engine" light circuit. Published diagnostic procedures address these conditions and give detailed steps to identify their cause. These procedures begin with the diagnostic circuit check.

Finally, there is the condition of an intermittent "check engine" light with no stored codes. This condition may be caused by intermittent grounds, feeds, electrical interference from ignition or charging system, etc., intermittent shorts in five (5) volt or eight (8) volt reference circuits including sensors or miscellaneous other electrical inputs.

In all cases of "check engine" light, good connections are extremely important. This is true for connections at components as well as connections at each ECM terminal. Also, all system grounds must be clean and tight. If there is any question about the integrity of connections, the terminal should be removed from the plastic connector body and reshaped or replaced for better contact.

Technicians that take the time to understand the codes and the conditions required to set the codes will be better able to fix "check engine" light occurrences. They will be the technicians that "fix it right the first time."

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.