Possible Causes & Corrections

Use following procedures to determine possible causes of intermittent Malfunction Indicator Light (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 in 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 faulty 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 regulator 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 Malfunction Indicator Light (MIL) with no codes set in memory.
• Check for improper installation of electrical accessories such as auxiliary lights or 2-way radios.
• Ensure EST 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 module is connected to a good ground.
• 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.
• On vehicles not equipped with a driver information center, use scan tool to check for intermittent wiring problem. See SCAN TOOL USAGE under SELF-DIAGNOSTIC SYSTEM in appropriate SELF-DIAGNOSTICS article.