- Intermittent Malfunctions

DEFINITION
Problem may or may not turn "ON" the Malfunction Indicator Lamp (MIL) "Service Engine Soon" or store a Diagnostic Trouble Code (DTC).

NOTE: Do NOT use the 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.

1. Most intermittent problems are caused by faulty electrical connections or wiring. Perform careful visual/physical check as described at start of Computers and Controls/Testing and Inspection/Procedures/. Testing and Inspection

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.

2. If a visual/physical check does not find the cause of the problem, the vehicle can be driven with a voltmeter connected to a suspected circuit. A Tech 1 scan tool can also be used to help detect intermittent conditions. An abnormal voltage or Tech 1 scan tool reading, when the problem occurs, indicates the problem may be in that circuit.

3. Loss of DTC memory. To check, disconnect Throttle Position (TP) sensor and idle engine until the MIL "Service Engine Soon" comes "ON." DTC 22 should be stored and kept in memory when ignition is turned "OFF." If not, the Powertrain Control Module (PCM) connections or the PCM is faulty.

4. An intermittent MIL "Service Engine Soon" with no stored DTC may be caused by:
^ Ignition coil shorted to ground and arcing at spark plug wires or plugs.
^ The MIL "Service Engine Soon" wire to PCM shorted to ground (circuit 419).
^ Faulty PCM ground circuits.

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

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

7. Ignition Control (IC) wires should be kept away from alternator. Wire from PCM to ignition system (CKT453) should have proper connections.

8. Check for open diode across A/C compressor clutch, and for other open diodes.

9. If problem has not been found, refer to PCM Connector Symptoms Chart.