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 MANUALSGMC1994C3500 HD 6.5 F, AUTOMATICREPAIR AND DIAGNOSISENGINE PERFORMANCESYSTEMENGINE CONTROLS - TESTS W/CODES - 6.5L DIESELDIAGNOSTIC TROUBLE CODE (DTC) IDENTIFICATIONNOTES
1994 GMC C3500 HD 6.5 F, Automatic
Diagnostic Trouble Code (DTC) Identification: Notes
1994 GMC C3500 HD 6.5 F, AutomaticSECTION Notes
NOTE:
Trouble codes retrieved from PCM may be either engine or transmission related. Only engine-related codes are covered in this article. For transmission-related codes, see TESTS W/CODES - ELECTRONIC TRANSMISSION
article in the ENGINE PERFORMANCE section. See DIAGNOSTIC TROUBLE CODE (DTC) IDENTIFICATION table to determine if code is engine or transmission related.
- The DEC system codes indicate failure of a specific sensor and/or circuit. Sensor/circuit diagnosis may indicate replacement of PCM. DTC 51 indicates PROM is either improperly installed or has failed.
- If PCM is replaced at any point during diagnostic testing and if condition is still not corrected after replacing PCM, the following may be the cause:
- An incorrect PCM or PROM application may cause a malfunction, which may or may not set a code.
- The PCM connector may be the problem. Connector terminals may have to be removed from connector to be checked properly.
- PROM failure. Although the PROM rarely fails, it could be the cause of the problem.
- Replacement PCM may be faulty.
- Intermittent problem. Make a careful physical inspection of affected sensor/circuit.
- A shorted solenoid, coil relay, or harness may be the cause of PCM failure. Use Short Circuit Tester (J-34636) to check for short circuits.
DIAGNOSTIC TROUBLE CODE (DTC) IDENTIFICATION
| Code | Probable Cause | ||||
|---|---|---|---|---|---|
| 13 (1) | Engine Shutoff Solenoid Circuit Fault | ||||
| 14 (1) | ECT Sensor Circuit Low (High Temp. Indicated) | ||||
| 15 (1) | ECT Sensor Circuit High (Low Temp. Indicated) | ||||
| 16 (1) | Vehicle Speed Sensor Buffer Fault | ||||
| 17 (1) | High Resolution Circuit Fault | ||||
| 18 (1) | Pump Cam Reference Pulse Error | ||||
| 19 (1) | Crankshaft Position Reference Error | ||||
| 21 (1) | Accelerator Pedal Position No. 1 Circuit High | ||||
| 22 (1) | Accelerator Pedal Position No. 1 Circuit Low | ||||
| 23 (1) | Accelerator Pedal Position No. 1 Circuit Range Fault | ||||
| 24 (2) | Vehicle Speed Sensor Transmission Output Signal | ||||
| 25 (1) | Accelerator Pedal Position No. 2 Circuit High | ||||
| 26 (1) | Accelerator Pedal Position No. 2 Circuit Low | ||||
| 27 (1) | Accelerator Pedal Position No. 2 Circuit Range Fault | ||||
| 28 (2) | Trans. Range Pressure Switch | ||||
| 29 (1) | Glow Plug Relay Circuit Fault | ||||
| 31 (1) | EGR Control Pressure/BARO Sensor Circuit Low (High Vacuum) | ||||
| 32 (1) | EGR Circuit Error | ||||
| 33 (1) | EGR Control Pressure/BARO Sensor Circuit High (Low Vacuum) | ||||
| 34 (1) | Injection Timing Stepper Motor Circuit Fault | ||||
| 35 (1) | Injection Pulse Width Error (Response Time Short) | ||||
| 36 (1) | Injection Pulse Width Error (Response Time Long) | ||||
| 37 (2) | TCC Brake Switch On/Off | ||||
| 38 (2) | TCC Brake Switch On/Off | ||||
| 39 (2) | TCC Stuck Off | ||||
| 41 (1) | Brake Switch Circuit Fault | ||||
| 42 (1) | Fuel Temperature Circuit Low (High Temp. Indicated) | ||||
| 43 (1) | Fuel Temperature Circuit High (Low Temp. Indicated) | ||||
| 44 (1) | EGR Pulse Width Error | ||||
| 45 (1) | EGR Vent Error | ||||
| 46 (1) | MIL (SERVICE ENGINE SOON) Circuit Fault | ||||
| 47 (1) | IAT Sensor Circuit Low (High Temp. Indicated) | ||||
| 48 (1) | IAT Sensor Circuit High (Low Temp. Indicated) | ||||
| 49 (1) | SERVICE THROTTLE SOON Light Circuit Fault | ||||
| DTC 51 - PROM PROBLEM (FAULTY OR INCORRECT PROM) DTC 51 - PROM PROBLEM (FAULTY OR INCORRECT PROM) 51 (1) | PROM Error (Faulty Or Incorrect PROM) | ||||
| 52/53 (2) | System Voltage High | ||||
| 54 (1) | PCM Fuel Circuit Error | ||||
| 56 (1) | Injection Pump Calibration Resistor Error | ||||
| 57 (1) | PCM 5-Volt Shorted | ||||
| 58 (2) | Transmission Fluid Temperature High | ||||
| 59 (2) | Transmission Fluid Temperature Low | ||||
| 61 (1) | Turbo Boost Sensor Circuit High | ||||
| 62 (1) | Turbo Boost Sensor Circuit Low | ||||
| 63 (1) | Accelerator Pedal Position No. 3 Circuit High | ||||
| 64 (1) | Accelerator Pedal Position No. 3 Circuit Low | ||||
| 65 (1) | Accelerator Pedal Position No. 3 Circuit Range Fault | ||||
| 66 (2) | 3-2 Control Solenoid Circuit Fault | ||||
| 67 (2) | TCC Solenoid Circuit Check | ||||
| 68 (2) | Overdrive Ratio Error | ||||
| 69 (2) | Torque Converter Clutch Stuck On | ||||
| 71 (1) | Set/Coast Switch Fault | ||||
| 72 (2) | VSS Circuit Loss Transmission Output Signal | ||||
| 73 (2) | Pressure Control Solenoid (Current Error) | ||||
| 74 (2) | Trans. Input Speed Error | ||||
| 75 (2) | System Voltage Low | ||||
| 76 (1) | Resume/Accel Switch Fault | ||||
| 78 (1) | Wastegate Solenoid Fault | ||||
| 79 (2) | Transmission Fluid Temperature High | ||||
| 81 (2) | Transmission 2-3 Error | ||||
| 82 (2) | Transmission 1-2 Error | ||||
| 83 (2) | TCC Solenoid Circuit Fault | ||||
| 84 (1) | Accelerator Pedal Position Circuit Fault | ||||
| 85 (2) | Undefined Gear Ratio | ||||
| 86 (2) | Low Gear Ratio Error | ||||
| 87 (2) | High Gear Ratio Error | ||||
| 88 (1) | TDC Offset Error | ||||
| 91 (1) | Cylinder Balance Fault No. 1 Cylinder | ||||
| 92 (1) | Cylinder Balance Fault No. 2 Cylinder | ||||
| 93 (1) | Cylinder Balance Fault No. 3 Cylinder | ||||
| 94 (1) | Cylinder Balance Fault No. 4 Cylinder | ||||
| 95 (1) | Cylinder Balance Fault No. 5 Cylinder | ||||
| 96 (1) | Cylinder Balance Fault No. 6 Cylinder | ||||
| 97 (1) | Cylinder Balance Fault No. 7 Cylinder | ||||
| 98 (1) | Cylinder Balance Fault No. 8 Cylinder | ||||
| 99 (1) | Accelerator Pedal Position No. 2 (5-Volt Reference Fault) | ||||
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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.