Chevrolet vehicles manufactured before 1996 use an internal computer called an Electronic Control Module, and it oversees the automated engine diagnostic routines. A Chevrolet's ECM can sense problems as they present themselves, and it keeps an ongoing record. The ECM assigns each fault and malfunction a code, which corresponds with General Motors standardized descriptions. Some of these codes pertain to the ECM itself. If this module fails or works erratically, then the Chevrolet's whole diagnostic system becomes untrustworthy. Troubleshooting the module requires the same proceedure as pulling the vehicle's fault codes.
Open your Chevrolet's driver's side door and find the Assembly Line Data Link. The ALDL, for all General Motors vehicles, will be found directly below the steering column, in the center of the dash's underside.
Run a length of jumper wire between the ALDL's "A" and "B" ports. These two slots are on the ALDL's top row, and all the way to the right, at the end. The "A" and "B" ports are also side by side.
Turn the Chevy's electrical system on, but leave the engine off. An active electrical system means the ECM will also be up and running.
Count the flash codes and record them onto a sheet of paper. The check engine light will flash the codes at you, in a series of long and short pulses of light. For example, General Motor's code 86 will appear as eight long flashes, followed six shorter flashes. There will be a dark, silent pause between coding sets.
Turn your Chevrolet's electrical system off and exit the car. You will need to research the flash codes your copied down. Your Chevrolet's owner's manual will not offer these codes. If you do not wish to spend a lot of money, you can find coding definitions online. However, if you want a source with a stronger sense of authority, you should obtain a repair manual for your Chevrolet's model and year. A repair handbook will also tell you, in detail, how to remedy problems with the ECM,
Consider taking the vehicle to the a General Motors approved mechanic. The ECM is a single a component, located behind the dash on the passenger's side. Repair options are limited. Besides investing the ECM's external wiring, you can either reprogram it or replace it. Replacement modules are not widely sold, so it would best to consult a professional opinion regarding your exact model and year.