- Intermittent Malfunctions

DEFINITION
Problem may or may not turn "ON" the MIL (Service Engine Soon) or store a DTC.

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

Most intermittent problems are caused by faulty electrical connections or wiring. Perform the careful visual/physical check as described in Preliminary Checks.
Check for:
- Poor mating of the connector halves or a 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 ensure proper contact tension.
- Poor terminal to wire connection. This requires removing the terminal from the connector body to check. Refer to "Introduction" in SECTION 6E "Wiring Harness Service."

- If a visual/physical check does not find the cause of the problem, the vehicle can be driven with a J 39200 digital multimeter connected to a suspected circuit. A Tech 1 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 trouble code was stored for a circuit having a sensor, except for DTCs 42, 43, 44 and 45 substitute a known good sensor and recheck.

- Loss of diagnostic trouble code memory. To check, disconnect MAP sensor and idle engine until MIL (Service Engine Soon) comes "ON." DTC 32 should be stored, and kept in memory when ignition is turned "OFF." If not, the Powertrain Control Module (PCM) is faulty.
- An intermittent MIL (Service Engine Soon) with no stored diagnostic trouble code may be caused by:

- Ignition coil shorted to ground and arcing at spark plug wires or plugs.
- MIL (Service Engine Soon) wire to PCM shorted to ground (CKT 419).
- PCM grounds, refer to Powertrain Control Module / Service and Repair.

- Check for an electrical system interference caused by a defective relay, Powertrain Control Module (PCM) 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, cellular phones, etc.
- Ignition Control (IC) wires should be routed away from spark plugs, ignition wires, electronic ignition system, components and generator. Wire from PCM to ignition control module (CKT 453) should have a good connection.
- Check for open diode across A/C compressor clutch, and for other open diodes (refer to wiring diagrams).
- If problem has not been found, refer to PCM Connector Symptom Charts.
- Tech 1 PCM snapshot function may be very useful in diagnosing intermittent powertrain problems. Refer to Computers and Controls / Scan-Tool Diagnostics for further information.