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 139 (ENGINE CONTROL SYSTEM - SELF-DIAGNOSTICS)DIAGNOSTIC TESTSDTC P0171: FUEL TRIM SYSTEM LEANACTION TAKEN WHEN DTC SETS
1999 GMC Forward Control 5.7 R, Automatic
Action Taken When DTC Sets
1999 GMC Forward Control 5.7 R, AutomaticSECTION Action Taken When DTC Sets
WARNING: This page is about a different car, the 2003 Pontiac Vibe. However, it is still accessible from the selected car via links, so may be relevant.
- PCM illuminates Malfunction Indicator Light (MIL) after 2 consecutive ignition cycles in which the diagnostic runs with fault active.
- PCM stores conditions which were present when DTC set as freeze frame data.
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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.