The SERVICE ENGINE SOON Malfunction Indicator Light (MIL) located in the instrument cluster serves 2 main functions:
- The lamp indicates to the driver when a problem has occurred and the vehicle should be taken for service as soon as reasonably possible.
- The light may be used by technicians to monitor diagnostic trouble codes and/or open/closed loop engine operation, whenever the system is placed in the diagnostic mode.
To verify proper operation of the bulb and wiring, the lamp will illuminate when the ignition is first turned to ON , but the engine is not running. If the system is operating properly, the lamp will turn OFF once the engine is started.
If the MIL remains lit once the engine is started, the self-diagnostic system has detected a problem. If the problem goes away, the light will extinguish in 10 seconds (in most cases), but a diagnostic trouble code will remain in the PCM memory.
After repairs are made to the faulty system(s), it is necessary to make sure the PCM memory is cleared of any old diagnostic trouble codes. Removing the battery voltage to the PCM for a minimum of 30 seconds will clear all codes. This may be accomplished in various ways depending on how the vehicle is equipped. The PCM harness power feed may be disconnected at the positive battery terminal "pigtail." The fuse may be removed from the inline fuse holder which originates at the positive battery connection or from fuse block, as applicable. Also, the negative battery cable may be disconnected, but other on-board data such as the clock or radio presets will also be lost.
The PCM has a "learning" ability which allows it to make corrections for minor variations in the fuel system, in order to improve driveability. If the battery is disconnected to clear diagnostic codes or for safety during repairs, the "learning" process will reset and must begin again. A change may be noted in the vehicle's performance while the learning process begins. To "teach" the vehicle, make sure the engine is at normal operating temperature, then drive the vehicle at part throttle, with moderate acceleration and idle conditions, until normal performance returns.