- Intermittent Malfunctions

Definition: Problem may or may not turn "ON" the "Service Engine Soon" light or store a code.

DO NOT use Diagnostic Trouble Code (DTC) charts for intermittent problems. The fault must be present to locate the problem. If a fault is intermittent, use of DTC charts may result in replacement of good parts.

Most intermittent problems are caused by faulty electrical connections or wiring. Perform a careful visual/physical and check for:
- Poor mating of the connector halves or terminal not fully seated in the connector body (backed out).
- Improperly formed or damaged terminal. All connector terminals in the problem circuit should be carefully reformed or replaced to insure proper contact tension.
- Poor terminal to wire connection. This requires removing the terminal from the connector body to check.

If a visual/physical check does not find the cause of the problem, the car can be driven with a voltmeter connected to a suspected circuit. A "Scan" tool can also be used to help detect intermittent conditions. An abnormal voltage, or "Scan" reading, when the problem occurs, indicates the problem may be in that circuit. If the wiring and connectors check OK, and a diagnostic code was stored for a circuit having a sensor, except for Codes 43, 44 and 45, substitute a known good sensor and recheck.

Loss of diagnostic code memory. To check, disconnect TPS and idle engine until "Service Engine Soon" light comes "ON." Code 22 should be stored, and kept in memory when ignition is turned "OFF." If not, the ECM is faulty.

An intermittent "Service Engine Soon" light with no stored diagnostic code may be caused by:
- Ignition coil shorted to ground and arcing at spark plug wires or plugs.
- "Service Engine Soon" light wire to ECM shorted to ground (CKT 419).
- Diagnostic "test" terminal wire to ECM, shorted to ground (CKT 451).
- ECM grounds, refer to ECM wiring diagrams.

Check for an electrical system interference caused by a defective relay, ECM driven solenoid, or switch. They can cause a sharp electrical surge. Normally, the problem will occur when the faulty component is operated.

Check for improper installation of electrical options such as lights, 2 way radios, etc.

EST wires should be routed away from spark plug wires, ignition coils, and generator. Wire from ECM to ignition module (CKT 453) should have a good connection.

Check for open diode between A/C compressor clutch CKTs 59 and 150 and for other open diodes (refer to wiring diagrams).

If problem has not been found, refer to ECM Connector Symptoms Chart. Pinout Values and Diagnostic Parameters