- 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 diagnostic trouble code charts may result in replacement of good parts.

^ Most intermittent problems are caused by: faulty electrical connections or wiring. Perform careful visual / physical check as described in Diagnostic Notes. See Diagnosis By Symptom / Preliminary CHecks. - Important Preliminary Checks

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 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 display, when the problem occurs, indicates the problem may be in that circuit.

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

^ An intermittent Malfunction Indicator Lamp (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.
- Malfunction Indicator Lamp (MIL) "Service Engine Soon" wire to PCM shorted to ground (CKT 419).
- PCM grounds, refer to Powertrain Control Module / Diagrams.

^ MIL (Service Engine Soon) wire to PCM shorted to ground (CKT 419).
- Diagnostic "test" terminal wire to PCM shorted to ground (CKT 451).
- PCM grounds, refer to Powertrain Control Module / Diagrams.
- Excessive current draw from faulty systems such as damaged cooling fan motor, etc.

^ 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.

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

^ Ignition control wires should be routed away from spark plug wires, Electronic Ignition (EI) system wires, ignition control module and generator. Wires from PCM to ignition control module should have a good connection

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

^ If the problem has not been found, refer to Diagnosis By Symptom / PCM Connector Symptoms Chart. Pinout Values and Diagnostic Parameters