MALFUNCTION INDICATOR LIGHT (MIL)
The Electronic Control Module (ECM) is equipped with a self-diagnostic system, which detects system failures or abnormalities. When a malfunction is detected during 2 sequential drive cycles, the ECM will illuminate the MIL on the instrument panel, and a corresponding Diagnostic Trouble Code (DTC) will be stored in ECM. To retrieve stored DTC(s), see RETRIEVING DIAGNOSTIC CODES . Malfunctions are recorded as hard failures or as intermittent failures. As a bulb check, Malfunction Indicator Light (MIL) will also illuminate for about 5 seconds when ignition is turned on. MIL should illuminate continuously when ignition is in ON position and engine is off.
Hard Failures
Hard failures cause the MIL to remain on until the problem is repaired and DTC is cleared. If the light comes on and remains on during vehicle operation, retrieve code(s). See RETRIEVING DIAGNOSTIC CODES to determine cause of malfunction.
Intermittent Failures
Intermittent failures may cause the MIL to flicker on and off after the fault goes away. The corresponding DTC will be retained in ECM memory. If the related fault does not reoccur within 3 subsequent driving cycles, the related DTC may be erased from ECU memory. Intermittent failures may be caused by sensor, connector, or wiring problems. See INTERMITTENTS in TROUBLE SHOOTING - NO CODES article.
SERVICE PRECAUTIONS
Before proceeding with diagnosis, following precautions must be observed
- Ensure vehicle has a fully charged battery and functional charging system.
- Visually inspect connectors and circuit wiring being worked on.
- DO NOT disconnect battery or ECM unless instructed to do so. Doing so will erase any fault codes stored in ECM.
- DO NOT cause short circuits when performing electrical tests. This will set additional fault codes, making diagnosis of original problem more difficult.
- DO NOT use a test light in place of a voltmeter.
- When checking for spark, DO NOT hold coil wire more than 1/4" from ground. Damage to vehicle electronics and/or ECM may result.
- DO NOT prolong testing of fuel injectors. Engine may hydrostatically (liquid) lock.
- When a vehicle has multiple fault codes, always repair lowest number fault code first, then repair codes in ascending numerical order.
ECM POWER & GROUND CIRCUITS
Proper power and ground supply to ECM is required to guarantee correct operation of ECM and self-diagnostic system. See ELECTRONIC CONTROL MODULE (ECM) under COMPUTERIZED ENGINE CONTROLS in SYSTEM & COMPONENT TESTING article.
SUMMARY
If no hard fault codes (or only pass codes) exist, driveability symptoms exist, or intermittent codes exist, proceed to TROUBLE SHOOTING - NO CODES article for diagnosis by symptom (i.e., ROUGH IDLE, NO START, etc.) or intermittent diagnostic procedure.