Possible Causes & Corrections
To determine possible causes of an intermittent SERVICE ENGINE SOON light 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 wire from SERVICE ENGINE SOON light to ECM for short to ground.
- Check wire from ALDL "test" terminal for intermittent short to ground.
- Check for poor connections in ECM ground terminals.
- Check for loss of trouble code memory. To check code, disconnect TPS, and run engine at idle until SERVICE ENGINE SOON light comes on. Code 22 should be stored and retained in memory when ignition is turned off. If not, ECM is faulty.
- Check for electrical system interference caused by a defective relay or an ECM-driven solenoid or switch which may cause a 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 manufacturers' specifications. Solenoids without original- equipment diodes for circuit protection and HEI-EST module or voltage regulator using transistors instead of silicone-chip circuitry may possibly cause voltage surges (up to 300 volts) in ECM wiring, causing temporary ECM shutdown. ECM shutdown is a normal response to system overvoltage (over 16 volts on most models). ECM repowers when condition ceases to exist. A rapid shutdown and repower could cause a flickering SERVICE ENGINE SOON light 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 ECM to distributor or ignition module is connected to a good ground.
- Check for intermittent short to ground on terminal "B" (diagnostic enable) of ALDL or in SERVICE ENGINE SOON light circuit.
- On vehicles not equipped with a driver information center, use scan tester to check for intermittent wiring problem. See SCAN TESTER USAGE in appropriate TESTS W/CODES article in this section.