Intermittent Problem Diagnosis

Intermittent problem may or may not illuminate Malfunction Indicator Light (MIL) or set a Diagnostic Trouble Code (DTC). DO NOT use Diagnostic Trouble Code (DTC) procedures to diagnose intermittent problems. Intermittent fault testing requires duplicating circuit or component failure to identify problem. These procedures may result in computer setting a fault code (on some systems) which may help in diagnosis.

Most intermittent problems are caused by faulty electrical connections or wiring. Ensure connector halves and terminals are fully seated, not damaged and not backed out. Ensure terminals are properly formed, and that there is a tight terminal-to-wire connection. Ensure proper terminal contact tension.

If problem vehicle does not produce fault codes, road test vehicle and monitor suspect circuit voltage or resistance values using a DVOM or scan tool. While attempting to reproduce conditions causing intermittent fault, a status change on DVOM or scan tool indicates a fault has been located.

Use a DVOM or scan tool to pinpoint faults. When monitoring voltage, ensure ignition switch is in ON position, or engine is running. Ensure ignition switch is in OFF position or negative battery cable is disconnected when monitoring circuit resistance. Status changes on DVOM or scan tool during test procedures indicate area or component of fault.

An intermittent MIL and no DTCs may be caused by electrical system interference from a defective relay, a PCM-driven solenoid or a switch. These defective components can cause a sharp electrical surge during system operation. Improper installation of add-on electrical devices, such as lights, radios, electric motors or communication devices. Ignition control wires improperly routed too close to spark plugs, ignition components and generator can also cause an intermittent MIL. Wire for engine ground from PCM to ignition system should be a zero loss ground. Ignition system secondary wiring or components shorting to ground. MIL circuit or Diagnostic Test terminal intermittently shorted to ground. Poor PCM grounds.

In order to check for loss of DTC memory, disconnect Throttle Position (TP) sensor and idle engine until MIL illuminates. A DTC should store and remain in PCM memory when ignition is turned off for at least 10 seconds. If a DTC is not stored, check for DTC P1510 or a faulty PCM.