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 MANUALSGMC1995FORWARD CONTROL 4.3 Z, STANDARDREPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCESYSTEMENGINE CONTROLS - TESTS W/CODES - 6.5L DIESEL.SELF-DIAGNOSTIC SYSTEMTROUBLE CODE IDENTIFICATION
1995 GMC Forward Control 4.3 Z, Standard
Trouble Code Identification
1995 GMC Forward Control 4.3 Z, StandardSECTION Trouble Code Identification
NOTE:
Trouble codes retrieved from PCM may be either engine or transmission related. Trouble codes retrieved from TCM are only transmission related. Both engine and transmission related codes are covered in this article. See the DTC IDENTIFICATION to determine if code is engine or transmission related.
- The DEC/electronically-controlled transmission systems codes indicate failure of a specific sensor and/or circuit. Sensor/circuit diagnosis may indicate replacement of PCM/TCM. DTC 51 indicates PROM is either improperly installed or has failed.
- If PCM/TCM is replaced at any point during diagnostic testing and if condition is still not corrected after replacing PCM/TCM, the following may be the cause:
- An incorrect PCM/TCM or PROM application may cause a malfunction, which may or may not set a code.
- The PCM/TCM 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/TCM 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/TCM failure. Use Short Circuit Tester (J-34636) to check for short circuits.
DTC IDENTIFICATION
| Code | Probable Cause | ||||
|---|---|---|---|---|---|
| DTC 13 (1) | Engine Shutoff Solenoid Circuit Fault | ||||
| DTC 14 (1) | ECT Sensor Circuit Low (High Temperature Indicated) | ||||
| DTC 15 (1) | ECT Sensor Circuit High (Low Temperature Indicated) | ||||
| DTC 16 (1) | Vehicle Speed Signal Buffer Fault | ||||
| DTC 17 (1) | High Resolution Circuit Fault | ||||
| DTC 18 (1) | Pump Cam Reference Pulse Error | ||||
| DTC 19 (1) | Crankshaft Position Reference Error | ||||
| DTC 21 (1) | APP No. 1 Circuit High (Models With PCM) | ||||
| DTCS 21/22 (2) | TP Sensor Circuit High/Low (4L80-E With TCM) | ||||
| DTC 22 (1) | APP No. 1 Circuit Low (Models With PCM) | ||||
| DTC 23 (1) | APP No. 1 Circuit Range Fault | ||||
| DTC 24 (2) | VSS Circuit Low (Transmission Output Speed Signal) | ||||
| DTC 25 (1) | Accelerator Pedal Position (APP) No. 2 Circuit High | ||||
| DTC 26 (1) | Accelerator Pedal Position (APP) No. 2 Circuit Low | ||||
| DTC 27 (1) | APP No. 2 Circuit Range Fault | ||||
| DTC 28 (2) | Transmission Range Pressure Switch Circuit Fault | ||||
| DTC 29 (1) | Glow Plug Relay Circuit Fault | ||||
| DTC 31 (1) | EGR Control Pressure/BARO Sensor Ckt Low (High Vac) | ||||
| DTC 32 (1) | EGR Circuit Error "G" Series | ||||
| DTC 33 (1) | EGR Control Pressure/BARO Sensor Ckt High (Low Vac) | ||||
| DTC 34 (1) | Injection Timing Stepper (ITS) Motor Circuit Fault | ||||
| DTC 35 (1) | Injection Pulse Width Error (Response Time Short) | ||||
| DTC 36 (1) | Injection Pulse Width Error (Response Time Long) | ||||
| DTCS 37/38 (2) | TCC Brake Switch Stuck On/Off | ||||
| DTC 39 (2) | TCC Stuck Off (4L80-E) | ||||
| DTC 41 (1) | Brake Switch Circuit Fault | ||||
| DTC 41 (1) | Brake Switch Circuit Fault - "P" Series | ||||
| DTC 42 (1) | Fuel Temp Circuit Low (High Temp Indicated) | ||||
| DTC 43 (1) | Fuel Temp Circuit High (Low Temp Indicated) | ||||
| DTC 44 (1) | EGR Pulse Width Error - "G" Series | ||||
| DTC 45 (1) | EGR Vent Error - "G" Series | ||||
| DTC 46 (1) | Malfunction Indicator Light (MIL) Circuit Fault | ||||
| DTC 47 (1) | IAT Sensor Circuit Low (High Temperature Indicated) | ||||
| DTC 48 (1) | IAT Sensor Circuit High (Low Temperature Indicated) | ||||
| DTC 49 (1) | SERVICE THROTTLE SOON Light Circuit Fault | ||||
| DTC 51 (1) | PROM Problem (Faulty Or Incorrect PROM) | ||||
| DTCS 52/53 (2) | System Voltage High Long | ||||
| DTC 54 (1) | PCM Fuel Circuit Error | ||||
| DTC 56 (1) | Injection Pump Calibration Resistor Error | ||||
| DTC 57 (1) | PCM 5-Volt Shorted | ||||
| DTC 58 (2) | Transmission Fluid Temp Sensor Circuit Low (High Temp Indicated) 4L60-E & 4L80-E | ||||
| DTC 59 (2) | Transmission Fluid Temp Sensor Circuit High (Low Temp Indicated) 4L60-E & 4L80-E | ||||
| DTC 61 (1) | Turbo Boost Sensor Circuit High - "P" Series | ||||
| DTC 62 (1) | Turbo Boost Sensor Circuit Low - "P" Series | ||||
| DTC 63 (1) | Accelerator Pedal Position (APP) No. 3 Circuit High (Models With PCM) | ||||
| DTCS 63/64 (2) | BARO Sensor Circuit High/Low (4L80-E With TCM) | ||||
| DTC 64 (1) | APP No. 3 Circuit Low (Models With PCM) | ||||
| DTC 65 (1) | APP No. 3 Circuit Range Fault | ||||
| DTC 66 (2) | 3-2 Control Solenoid Ckt Fault - "G" Series 4L60-E | ||||
| DTC 67 (2) | TCC Solenoid Circuit Fault - "G" Series 4L60-E | ||||
| DTC 68 (2) | Transmission Component Slipping (4L80-E) | ||||
| DTC 69 (2) | TCC Stuck On - "G" Series 4L60-E | ||||
| DTC 69 (1) | TCC Stuck On (4L80-E) | ||||
| DTC 71 (1) | Set/Coast Switch Fault (Models With PCM) | ||||
| DTC 71 (2) | Camshaft Position Sensor Circuit Low (4L80-E W/ TCM) | ||||
| DTC 72 (2) | Speed Sensor Circuit Loss (Trans Output Speed Signal) | ||||
| DTC 73 (2) | Pressure Control Solenoid Circuit (Current Error) | ||||
| DTC 74 (2) | Transmission Input Speed Sensor Circuit (4L80-E) | ||||
| DTC 75 (2) | System Voltage Low | ||||
| DTC 76 (1) | Resume/Accel Switch Fault | ||||
| DTC 78 (1) | Wastegate Solenoid Fault - "P" SERIES | ||||
| DTC 79 (2) | Transmission Fluid Overtemp - 4L60-E & 4L80-E | ||||
| DTC 81 (2) | 2-3 Shift Solenoid Circuit Fault | ||||
| DTC 82 (2) | 1-2 Shift Solenoid Circuit Fault | ||||
| DTC 83 (2) | TCC PWM Solenoid Circuit Fault | ||||
| DTC 84 (1) | Accelerator Pedal Position (APP) Circuit Fault | ||||
| DTC 85 (2) | Undefined Ratio Error (4L80-E) | ||||
| DTC 86 (2) | Low Ratio Error (4L80-E) | ||||
| DTC 87 (2) | High Ratio Error (4L80-E) | ||||
| DTC 88 (1) | TDC Offset Error | ||||
| DTC 91 (1) | Cylinder Balance Fault No. 1 Cylinder | ||||
| DTC 92 (1) | Cylinder Balance Fault No. 2 Cylinder | ||||
| DTC 93 (1) | Cylinder Balance Fault No. 3 Cylinder | ||||
| DTC 94 (1) | Cylinder Balance Fault No. 4 Cylinder | ||||
| DTC 95 (1) | Cylinder Balance Fault No. 5 Cylinder | ||||
| DTC 96 (1) | Cylinder Balance Fault No. 6 Cylinder | ||||
| DTC 97 (1) | Cylinder Balance Fault No. 7 Cylinder | ||||
| DTC 98 (1) | Cylinder Balance Fault No. 8 Cylinder | ||||
| DTC 99 (1) | APP 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.