Possible Cause & Correction

- To track down possible causes of an intermittent "SERVICE ENGINE SOON" light 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 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 memory on fuel injected models, 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. On carbureted engines, ground dwell lead for 10 seconds with ALDL "test" lead left ungrounded. Code 23 should be stored and retained in memory after 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 silicon-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 will repower when condition ceases to exist. This 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.
• Make sure that EST wires are kept away from spark plug wires, distributor wires, distributor housing, ignition coil and generator. Make sure that 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, check for intermittent wiring problem using "Scan" tester. See SCAN TESTER USAGE in the appropriate TESTS W/CODES article in this section.