Possible Cause & Correction

To track down possible causes of an intermittent MIL, check the following items:

• Check for poor mating of one connector to another. Terminals may not be fully seated. Check for improperly formed or damaged terminals. Check wire to terminal connections.
• Check for poor connection from ignition coil to ground or arcing at spark plug wires or plugs.
• Check for intermittent short to ground on data or UART circuits of Data Link Connector (DLC) or in Malfunction Indicator Light (MIL) circuit. See appropriate wiring diagram in WIRING DIAGRAMS article.
• Check for poor connections in PCM ground terminals.
• Check for loss of diagnostic trouble code memory. To check code memory on fuel injected models, disconnect MAP or TP sensor and run engine at idle until MIL comes on. MAP or TP sensor code should be stored and retained in memory when ignition is turned off. If code is not stored, PCM is faulty.
• Check for electrical system interference caused by a defective relay, or an PCM-driven solenoid or switch which may cause sharp electrical surge. This type of problem will normally occur when faulty component is operated.
• Check for aftermarket parts which may not have been produced to manufacturer's specifications.
• Check for any open diodes in A/C or engine wiring.
• Check for improper installation of electrical accessories such as auxiliary lights or 2-way radios.
• Ensure ignition control wires are kept away from spark plug wires, distributor wires, distributor housing, ignition coil and generator. Ensure ground wire from PCM to distributor or ignition control module is connected to a good ground.