MALFUNCTION INDICATOR LIGHT (MIL)
Vehicles are equipped with a Malfunction Indicator Light (MIL). As a bulb check, light will illuminate for about 5 seconds when ignition is turned on. The MIL will also illuminate when a system failure is detected and a corresponding Diagnostic Trouble Code (DTC) will set in ECM memory.
The Electronic Control Module (ECM) is equipped with a self-diagnostic system, which detects system failures or abnormalities. When a malfunction occurs, the ECM will illuminate the MIL on the instrument panel and a corresponding DTC will be stored in ECM. To retrieve stored DTC(s), see RETRIEVING CODES. Malfunctions are recorded as hard failures or as intermittent failures.
Hard Failures
Hard failures cause the MIL to remain on until the problem is repaired. If the light comes on and remains on during vehicle operation, retrieve code(s). See RETRIEVING 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 TESTS W/O 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.
RETRIEVING CODES
Note. For specific scan tool operating procedures, see the scan tool manufacturer's instruction manual.
- Before entering on-board diagnostics, refer to SERVICE PRECAUTIONS. Turn ignition off. Locate Data Link Connector (DLC). Connect scan tool. (Scheme 3)
- Operate scan tool according to manufacturer's instructions. Read and record scan tool self-diagnostic output. Perform necessary repair(s). See DIAGNOSTIC TROUBLE CODE (DTC) DEFINITIONS and appropriate DIAGNOSTIC TESTS.
Scheme 3
CLEARING CODES
After testing and repairs have been completed, disconnect negative battery cable for at least 15 seconds to clear codes.
ECM POWER & GROUND CIRCUITS
See ELECTRONIC CONTROL MODULE (ECM) under COMPUTERIZED ENGINE CONTROLS in SYSTEM COMPONENT TESTS article.
SUMMARY
If no hard fault codes (or only pass codes) exist, driveability symptoms exist, or intermittent codes exist, proceed to TESTS W/O CODES article for diagnosis by symptom (i.e., ROUGH IDLE, NO START, etc.) or intermittent diagnostic procedures.