System Testing: Notes

A Check Engine Light (CEL) will come "ON" to indicate that a fault has occurred in the system. This light will not flash nor will the ECM store a fault code. The type of fault may be determined by the operation of the CEL. The light will come "ON" steady to indicate a vacuum fault or engine speed of less than 400 RPM.

If a vacuum fault is recognized, the ECM will turn the EGR valve "OFF." Vacuum faults include EPR, EGR and MAP systems. The CEL may also be "ON" under certain load and RPM conditions. When the CEL is "throttle position" related, the fault will be in the engine speed, VSS or MVS systems.

It takes the ECM approximately 10 seconds to recognize a fault except for an engine speed of less than 400 RPM which is recognized immediately. If a vacuum error is detected by the MAP sensor, CEL will come on and stay on until the ignition is turned off and on, even if the vacuum error is corrected. All other faults detected by the ECM that cause the CEL to light will also allow CEL to turn off if the fault is repaired or intermittent.

Use appropriate trouble shooting charts, wiring diagrams and explanatory text to diagnose faults indicated by the CEL. Numbered steps in the text correspond to the numbers in the related charts. Wiring diagrams are provided for further clarification.