Diagnostic System Operation
NOTE:
On carbureted models, a "SERVICE ENGINE SOON" light driver module is installed in wiring harness from ECM to "SERVICE ENGINE SOON" light. This driver turns on light when ignition is turned on. When vehicle starts, ECM turns light off. If ECM malfunctions or senses a malfunction, light will turn back on. On fuel injected models, driver is an integral part of ECM and is not serviceable.
The ECM is equipped with a self-diagnostic system which detects system failures or abnormalities. As a bulb and system check, "SERVICE ENGINE SOON" light will glow when ignition switch is turned to "ON" position and engine is not running. When engine is started, light should go out. If not, a malfunction has been detected in the computerized engine control system or "SERVICE ENGINE SOON" light circuit is faulty.
When a malfunction occurs, ECM will illuminate the "SERVICE ENGINE SOON" light located on instrument panel. When malfunction is detected and light is turned on, a corresponding trouble code will be stored in ECM memory. Malfunctions are recorded as "hard failures" or as "intermittent failures".