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 DIAGNOSISENGINE PERFORMANCETESTING & DIAGNOSISENGINE CONTROL SYSTEM & COMPONENT TESTSFUEL SYSTEM (DIESEL)GLOW PLUG SYSTEM DIAGNOSIS
1999 GMC Forward Control 5.7 R, Automatic
Glow Plug System Diagnosis
1999 GMC Forward Control 5.7 R, AutomaticSECTION Glow Plug System Diagnosis
- If On-Board Diagnostic (OBD) system check has been performed, go to next step. If On-Board Diagnostic (OBD) system check has not been performed, perform On-Board Diagnostic (OBD) system check. See ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK in appropriate SELF-DIAGNOSTICS article.
- Retrieve DTCs. If DTC P0117, P0118 and/or P0380 is not set, go to next step. If DTC P0117, P0118 and/or P0380 is set, perform appropriate DTC test. See appropriate SELF-DIAGNOSTICS article.
- Turn ignition off. Disconnect all glow plug harness connectors. Using a test light connected to battery voltage, probe each glow plug spade terminal at glow plugs. If test light illuminates at each glow plug, go to next step. If test light does not illuminate at each glow plug, go to step 9.
- Ensure all glow plug harness connectors are still disconnected. Turn ignition on with engine off. Using scan tool, command glow plugs on. Using a test light connected to ground, probe each glow plug harness connector terminal. If test light illuminates at each glow plug harness connector, go to next step. If test light does not illuminate at each glow plug harness connector, go to step 10.
- Using scan tool, command glow plugs off. Using a test light connected to ground, probe each glow plug harness connector terminal. If test light illuminates at any glow plug harness connector, go to next step. If test light does not illuminate at any glow plug harness connector, go to step 11.
- Remove glow plug relay. Using scan tool, command glow plugs off. Using a test light connected to ground, probe each glow plug harness connector terminal. If test light illuminates at any glow plug harness connector, go to next step. If test light does not illuminate at any glow plug harness connector, go to step 8.
- Repair short to voltage in glow plug output circuit or glow plug signal circuit. After repairs, go to step 11.
- Replace glow plug relay. After repairs, go to step 11.
- Replace glow plugs that did not illuminate test light. After repairs, go to step 11.
- Repair open in glow plug circuit(s) that did not illuminate test light. After repairs, go to next step.
- Operate vehicle under conditions when problem was noted. If system does not operate properly, go to step 1.
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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.