CHECK ENGINE LIGHT OPERATION EXPLANATION & CLARIFICATION

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 vehicles for three 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 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 10 seconds after the fault is gone. However, the trouble code will remain in the ECM memory until battery power to the ECM in interrupted. This battery power is supplied on most vehicles, 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, 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, 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, 55) will turn "ON" the "Check Engine" light within 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 corrects itself, the "Check Engine" light will go off, but the trouble code will be stored.

One 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 vehicle must be run for two minutes at curb idle. This allows codes that can set at idle to set. Intermittent conditions or codes that require driving the truck are covered in the specific code charts.

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.