Possible Causes & Corrections

Use the following procedures to determine possible causes of intermittent MIL operation:

• 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 poor connections at Powertrain Control Module (PCM) ground terminals.
• Check for electrical system interference caused by defective relay, PCM-driven solenoid or switch, which may cause sharp electrical surge. This type of problem will normally occur when problem component is operated.
• Check for aftermarket parts which may not have been produced to manufacturer's specifications. Solenoids without original equipment diodes for circuit protection or voltage regulators using transistors instead of silicone-chip circuitry may cause voltage surges (up to 300 volts) in PCM wiring, causing temporary PCM shutdown. PCM shutdown is a normal response to system over voltage (over 16 volts on most models). PCM re-powers when condition ceases to exist. A rapid shutdown and re-power could cause a flickering MIL with no DTCs set in memory.
• Check for improper installation of electrical accessories such as auxiliary lights, cell phones or 2-way radios.
• Ensure ground wire from PCM to distributor or ignition module is connected to a good ground.
• Check for intermittent short to ground on data circuits of data link connector or in MIL circuit. See ENGINE PERFORMANCE article in WIRING DIAGRAMS.
• On vehicles not equipped with a driver information center, use scan tool to check for intermittent wiring problem. See appropriate SELF-DIAGNOSTICS article in ENGINE PERFORMANCE.