Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 78 (ENGINE CONTROL SYSTEM SELF-DIAGNOSTICS - 3.8L)SYSTEM TESTSMIL ALWAYS ONDIAGNOSTIC PROCEDURE
1999 GMC Forward Control 5.7 R, Automatic
Diagnostic Procedure
1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedure
WARNING: This page does not describe the selected car, but rather 7 other vehicles, including the 2002 Pontiac Grand Prix, 2002 Pontiac Bonneville, 2002 Chevrolet Monte Carlo, 2002 Chevrolet Impala, and 2002 Buick Regal. However, it is still accessible from the selected car via links, so may be relevant.
- Perform Diagnostic System Check - Engine Controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS under SELF-DIAGNOSTIC SYSTEM. After performing Diagnostic System Check - Engine Controls, go to next step.
- Turn ignition on, engine off. If MIL turns off when ignition is turned on, go to step 5. If MIL does turn off when ignition is turned on, go to next step.
- Turn ignition off. Disconnect PCM. Turn ignition on, engine off. Observe MIL. If MIL is on, go to next step. If MIL is off, go to step 7.
- Remove IPC. Check Brown/White wire between PCM and IPC for a short to ground. If problem was found, repair as necessary. After repairs, go to step 8. If problem was not found, go to step 6.
- Repair short to voltage in Brown/White wire between IPC and PCM. After repairs, go to step 8.
- Replace IPC. After repairs, go to step 8.
- Replace PCM. After repairs, go to next step.
- Operate system to verify repair.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.