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 89 (ENGINE CONTROLS - SELF-DIAGNOSTICS - 4.0L)DIAGNOSTIC TESTSDTC P0650: MALFUNCTION INDICATOR LIGHT CONTROL CIRCUITDIAGNOSTIC PROCEDURES
1999 GMC Forward Control 5.7 R, Automatic
Diagnostic Procedures
1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedures
WARNING: This page is about a different car, the 2002 Oldsmobile Aurora. However, it is still accessible from the selected car via links, so may be relevant.
- Did you perform the Diagnostic System Check - Engine Controls? If yes, go to next step. If no, go to DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS under SELF-DIAGNOSTIC SYSTEM.
- Is DTC P1634 also set? If yes, see DTC P1634: IMPROPER IGNITION 1 INPUT . If no, go to next step.
- Turn the ignition ON, with the engine OFF. Observe the Malfunction Indicator Lamp (MIL) display on the Instrument Panel Cluster (IPC). Is the MIL ON? If yes, go to next step. If no, go to step 6.
- Command the MIL OFF with a scan tool. Is the MIL OFF? If yes, see INTERMITTENT CONDITIONS under SELF-DIAGNOSTIC SYSTEM. If no, go to next step.
- Test the MIL control circuit for a short to ground. If a condition is found, repair as necessary. Did you find and correct the condition? If yes, go to step 14. If no, go to step 12.
- Turn the ignition OFF. Disconnect the PCM connector. Connect the test lamp between the MIL control circuit and ground. Turn the ignition ON, with the engine OFF. Is the test lamp ON? If yes, go to next step. If no, go to step 8.
- Test the MIL control circuit for a short to voltage. If a condition is found, repair as necessary. Did you find and correct the condition? If yes, go to step 14. If no, go to step 11.
- Turn the ignition OFF. Connect the PCM connector. Disconnect the IPC connector. Connect the test lamp between the battery voltage and the MIL control circuit. Use the scan tool in order to command the MIL ON and OFF. Does the test lamp turn ON and OFF? If yes, go to next step. If no, go to step 10.
- Inspect for poor connections at the IPC harness connector. If a condition is found, repair as necessary. Did you find and correct the condition? If yes, go to step 14. If no, go to step 13.
- Test for an open MIL control circuit. If a condition is found, repair as necessary. Did you find and correct the condition? If yes, go to step 14. If no, go to next step.
- Inspect for poor connections at the PCM. If a condition is found, repair as necessary. Did you find and correct the condition? If yes, go to step 14. If no, go to next step.
- Replace the PCM. See appropriate REMOVAL, OVERHAUL & INSTALLATION article. Reprogram PCM. See POWERTRAIN CONTROL MODULE under PROGRAMMING. After repairs, go to step 14.
- Replace the IP cluster. See REMOVAL & INSTALLATION in appropriate ANALOG INSTRUMENT PANELS article in ACCESSORIES & EQUIPMENT. After repairs, go to next step.
- Use the scan tool in order to clear the DTCs. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions for Running DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step 2.
- With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, system is okay.
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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.