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.
1994 GMC Forward Control 7.4 NFACTORY OEM SERVICE PROCEDURE
Engine Controls - Tests W/Codes - 7.4L
- Introduction
- Self-Diagnostic System
- Trouble Code Charts
- Notes
- Code 13, Open Oxygen Sensor Circuit
- Code 14, Coolant Sensor Signal Voltage Low
- Code 15, Coolant Sensor Signal Voltage High
- Code 16, VSS Buffer Fault - "G" Series
- Code 21, TPS Signal Voltage High
- Code 22, TPS Signal Voltage Low
- Code 24, VSS - "C" & "K" Series, "P" Series M/T
- Code 24, VSS Trans Output Signal - "G" Series, "P" Series M/T
- Code 32, EGR Circuit Error With Linear EGR
- Code 32, EGR Circuit Error - "C" & "K" Series M/T
- Code 32, EGR CKT Err - Using Manifold Vac, 2-Terminal Solenoid & BACKPRESS EGR
- Code 32, EGR CKT Error Models Using Ported Vacuum & EVRV Sol
- Code 33, Map Sensor Signal Voltage High
- Code 34, Map Sensor Signal Voltage Low
- Code 35, Idle Speed Error
- Code 36, Idle SPD Actuator Fault - "C", "K" & "P" Series A/T
- Code 42 - Ignition Control
- Code 43, Knock Sensor W/O Spark Module (Single Sensor) - A/T
- Code 43, Knock Sensor With Spark Control Module
- Code 44, Lean Exhaust Indication
- Code 45, Rich Exhaust Indication
- Code 51, Prom Error (Faulty Or Incorrect Prom)
- Code 52, Faulty CALPAK
- Code 53, System OVERVOLTAGE
- Code 55, ECM/Pcm Error
- Code 54, Fuel Pump Circuit
- Summary
Factory Service Tools
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
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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.
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