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 MANUALSGMC1991FORWARD CONTROL 5.7 K, AUTOMATICREPAIR AND DIAGNOSISENGINE PERFORMANCESYSTEMENGINE CONTROLS - TESTS W/CODES - GASOLINE
1991 GMC Forward Control 5.7 K, Automatic
Engine Controls - Tests W/Codes - Gasoline
1991 GMC Forward Control 5.7 K, AutomaticSECTION Engine Controls - Tests W/Codes - Gasoline
- Introduction
- Self-Diagnostic System
- Trouble Code Charts
- Notes
- Code 13, Open Oxygen Sensor Circuit (Except 4.3L Turbo)
- Code 13, Open Oxygen Sensor Circuit (4.3L Turbo)
- Code 14, Coolant Sensor Signal Voltage Low
- Code 15, Coolant Sensor Signal Voltage High
- Code 21, TPS Signal Voltage High
- Code 22, TPS Signal Voltage Low
- Code 23, Mat Sensor Temp. Low (2.5L & 4.3L Turbo)
- Code 24, Vehicle Speed Sensor (Except 3.1L, C & K Series & Models W/4L80-E Transmission)
- Code 24, Vehicle Speed Sensor (C & K Series Except W/4L80-E Transmission)
- Code 24, Vehicle Speed Sensor (C & K Series 2WD W/4L80-E Transmission)
- Code 24, Vehicle Speed Sensor (2WD W/4L80-E Trans. Except C & K Series)
- Code 24, Vehicle Speed Sensor (4WD With 4L80-E Transmission)
- Code 24, Vehicle Speed Sensor (3.1L Engine)
- Code 25, Mat Sensor Temp. High (2.5L & 4.3L Turbo)
- Code 31, Turbo OVERBOOST (4.3L Turbo)
- Code 32, EGR System Error (4.3L C, G, K, & P Series W/4L80-E Transmission)
- Code 32, EGR System Error (All W/4L80-E Trans. Except 4.3L)
- Code 32, EGR System Error (All W/O 4L80-E Trans Except 3.1L, & 4.3L Turbo)
- Code 32, EGR System Error (3.1L Engine)
- Code 32, EGR System Error (4.3L Turbo)
- Code 33, Map Sensor Signal Voltage High
- Code 34, Map Sensor Signal Voltage Low
- Code 35, Idle Speed Error 2.5L & 4.3L Turbo
- Code 42, Electronic Spark Timing (Est)
- Code 43, ESC (All Except 4.3L Turbo & Models W/4L80-E Trans)
- Code 43, ESC (4.3L Turbo & Models W/ 4L80-E Trans)
- Code 44, Lean Exhaust Indication (Except 4.3L Turbo)
- Code 44, Lean Exhaust Indication (4.3L Turbo)
- Code 45, Rich Exhaust Indication (Except 4.3L Turbo)
- Code 45, Rich Exhaust Indication (4.3L Turbo)
- Code 51, Faulty Prom/Mem-Cal
- Code 52, Faulty CALPAK (Except 2.5L)
- Code 53, System OVERVOLTAGE (2.5L)
- Code 54, Fuel Pump Circuit
- Code 55, ECM Error (Except 2.5L)
- Summary
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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.