Intermittent Check Engine Light

An intermittent check engine light comes on at times, but does not stay on. A trouble code may be stored, but will erase after key is turned off and engine is restarted. Do not use trouble code charts to trouble shoot intermittent check engine light faults. This may result in replacement of good parts.

Use the following procedure to trouble shoot intermittent check engine light faults:

1. Perform careful visual inspection of vacuum hoses, checking for splits, kinks, deterioration, and proper connections as shown on emission control information label. Check fuel injection system for leaks.
2. Check for backed out connector halves (not fully seated), damaged or collapsed terminal connectors, and broken wires.
3. If visual examination does not reveal problem, connect voltmeter to suspected circuit and ground. Drive car and watch voltmeter to see if it changes at same moment that problem is noticed.
4. Check for intermittent ground in circuit between check engine light wire and ECM, between diagnostic test terminal on ALCL and ECM, and between ECM Terminal "K" and engine ground. Also see Chart A1, "CHECK ENGINE" LIGHT MALFUNCTION in this article.
5. Check for electrical system interference caused by defective relay, ECM driven solenoid or switch.
6. Check for improper installation of electrical options, including lights and 2-way radios.
7. Check for open diode or resistor across A/C compressor clutch. Check wiring diagram for other open diodes.