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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 235 (ENGINE CONTROLS - THEORY & OPERATION - 3.4L)SELF-DIAGNOSTIC SYSTEMSMALFUNCTION INDICATOR LIGHT
1999 GMC Forward Control 5.7 R, Automatic

Malfunction Indicator Light

1999 GMC Forward Control 5.7 R, AutomaticSECTION Malfunction Indicator Light
WARNING: This page is about a different car, the 2003 Pontiac Grand Am, 2003 Oldsmobile Alero, 2003 Chevrolet Monte Carlo, and 2003 Chevrolet Impala. However, it is still accessible from the selected car via links, so may be relevant.

The Malfunction Indicator Light (MIL) is located in the instrument panel cluster. The MIL will display as either SERVICE ENGINE SOON or a engine symbol icon when commanded ON.

The MIL indicates that an emissions related fault has occurred and vehicle service is required. The following is a list of the modes of operation for the MIL:

  • The MIL illuminates when the ignition switch is turned to RUN position, with the engine OFF. This is a bulb test to ensure the MIL is able to illuminate.
  • The MIL turns OFF after the engine is started if a diagnostic fault is not present.
  • The MIL remains illuminated after the engine is started if the Powertrain Control Module (PCM) detects a fault. A Diagnostic Trouble Code (DTC) is stored any time the PCM illuminates the MIL due to an emissions related fault. The MIL turns OFF after three consecutive ignition cycles in which a Test Passed has been reported for the diagnostic test that originally caused the MIL to illuminate.
  • The MIL flashes if the PCM detects a misfire condition which could damage the catalytic converter.
  • When the MIL is illuminated and the engine stalls, the MIL will remain illuminated as long as the ignition switch is in the RUN position.
  • When the MIL is not illuminated and the engine stalls, the MIL will not illuminate until the ignition is cycled OFF and then ON.

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Stop DIY work and contact a certified mechanic immediately if any of the following apply:

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  • • The engine overheats, stalls repeatedly, or misfires under load.
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