Diagnostic System Operation: Notes
NOTE:
On EFI models, the "SERVICE ENGINE SOON" lamp driver is built into the ECM. This driver turns on light when ignition is turned on. When vehicle starts, ECM turns light off. If ECM malfunctions or senses malfunction, light will turn back on.
The ECM of computerized engine control system 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".