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.
1998 GMC Forward Control 6.5 FFACTORY OEM SERVICE PROCEDURE
Diagnostic Trouble Codes
DIAGNOSTIC TROUBLE CODES
| Code | Probable Cause | ||||
|---|---|---|---|---|---|
| P0101 (1) | MAF System Performance | ||||
| P0102 (1) | MAF Sensor Circuit Low Frequency | ||||
| P0102 (1) | ("C" & "K" Series) | ||||
| P0103 (1) | MAF Sensor Circuit High Frequency | ||||
| P0103 (1) | ("C" & "K" Series) | ||||
| P0112 (1) | IAT Sensor Circuit Low Voltage | ||||
| P0113 (1) | IAT Sensor Circuit High Voltage | ||||
| P0117 (1) | ECT Sensor Circuit Low Voltage | ||||
| P0118 (1) | ECT Sensor Circuit High Voltage | ||||
| P0121 (1) | APP Sensor 1 Circuit Performance | ||||
| P0122 (1) | APP Sensor 1 Circuit Low Voltage | ||||
| P0123 (1) | APP Sensor 1 Circuit High Voltage | ||||
| P0126 (1) | Insufficient ECT For Stable Operation | ||||
| P0126 (1) | ("C" & "K" Series) | ||||
| P0182 (1) | Fuel Temperature Sensor Circuit Low Voltage | ||||
| P0183 (1) | Fuel Temperature Sensor Circuit High Voltage | ||||
| P0215 (1) | Engine Shutoff Solenoid Control Circuit | ||||
| P0216 (1) | Injection Timing Control System | ||||
| P0218 (2) | Transmission Fluid Overtemperature | ||||
| P0219 (1) | Engine Overspeed Condition | ||||
| P0220 (1) | APP Sensor 2 Circuit | ||||
| P0221 (1) | APP Sensor 2 Circuit Performance | ||||
| P0222 (1) | APP Sensor 2 Circuit Low Voltage | ||||
| P0223 (1) | APP Sensor 2 Circuit High Voltage | ||||
| P0225 (1) | APP Sensor 3 Circuit | ||||
| P0226 (1) | APP Sensor 3 Circuit Performance | ||||
| P0227 (1) | APP Sensor 3 Circuit Low Voltage | ||||
| P0228 (1) | APP Sensor 3 Circuit High Voltage | ||||
| P0231 (1) | Fuel/Lift Pump Feedback Circuit Low Voltage | ||||
| P0236 (1) | Turbocharger Boost System | ||||
| P0237 (1) | Turbocharger Boost Sensor Circuit Low Voltage | ||||
| P0238 (1) | Turbocharger Boost Sensor Circuit High Voltage | ||||
| P0251 (1) | Injection Pump Cam System | ||||
| P0263 (1) | Cylinder No. 1 Balance System | ||||
| P0266 (1) | Cylinder No. 2 Balance System | ||||
| P0269 (1) | Cylinder No. 3 Balance System | ||||
| P0272 (1) | Cylinder No. 4 Balance System | ||||
| P0275 (1) | Cylinder No. 5 Balance System | ||||
| P0278 (1) | Cylinder No. 6 Balance System | ||||
| P0281 (1) | Cylinder No. 7 Balance System | ||||
| P0284 (1) | Cylinder No. 8 Balance System | ||||
| P0300 (1) | Engine Misfire Detected | ||||
| P0301 (1) | Cylinder No. 1 Misfire Detected | ||||
| P0302 (1) | Cylinder No. 2 Misfire Detected | ||||
| P0303 (1) | Cylinder No. 3 Misfire Detected | ||||
| P0304 (1) | Cylinder No. 4 Misfire Detected | ||||
| P0305 (1) | Cylinder No. 5 Misfire Detected | ||||
| P0306 (1) | Cylinder No. 6 Misfire Detected | ||||
| P0307 (1) | Cylinder No. 7 Misfire Detected | ||||
| P0308 (1) | Cylinder No. 8 Misfire Detected | ||||
| P0335 (1) | CKP Sensor Circuit Performance | ||||
| P0370 (1) | Timing Reference High Resolution | ||||
| P0380 (1) | Glow Plug Circuit Performance | ||||
| P0400 (1) | EGR Flow Malfunction | ||||
| P0401 (1) | EGR Flow Insufficient | ||||
| P0402 (1) | EGR Flow Excessive | ||||
| P0404 (1) | EGR Flow Performance | ||||
| P0405 (1) | EGR Sensor Circuit Low Voltage | ||||
| P0406 (1) | EGR Sensor Circuit High Voltage | ||||
| P0501 (1) | Vehicle Speed Sensor Circuit | ||||
| P0560 (2) | System Voltage Malfunction | ||||
| P0567 (1) | Cruise Resume Circuit | ||||
| P0568 (1) | Cruise Set Circuit | ||||
| P0571 (1) | Cruise Brake Switch Circuit | ||||
| P0601 (1) | PCM Memory | ||||
| P0602 (1) | PCM Not Programmed | ||||
| P0604 (1) | Control Module Random Access Memory | ||||
| P0604 (1) | ("C" & "K" Series) | ||||
| P0606 (1) | PCM Internal Communication Interrupted | ||||
| P0711 (2) | TFT Sensor Circuit - Range/Performance | ||||
| P0712 (2) | TFT Sensor Circuit - Low Input | ||||
| P0713 (2) | TFT Sensor Circuit - High Input | ||||
| P0716 (2) | Input Speed Sensor Circuit - Range/Performance | ||||
| P0717 (2) | Input Speed Sensor Circuit - No Signal | ||||
| P0719 (2) | Brake Switch Circuit - Low Input | ||||
| P0722 (2) | Output Speed Sensor - Low Input | ||||
| P0723 (2) | Output Speed Sensor - Intermittent | ||||
| P0724 (2) | Brake Switch Circuit - High Input | ||||
| P0741 (2) | TCC Stuck Off | ||||
| P0742 (2) | TCC Stuck On | ||||
| P0748 (2) | PCS Circuit - Electrical | ||||
| P0751 (2) | 1-2 Shift Solenoid - Performance | ||||
| P0753 (2) | 1-2 Shift Solenoid - Electrical | ||||
| P0756 (2) | 2-3 Shift Solenoid - Performance | ||||
| P0758 (2) | 2-3 Shift Solenoid - Electrical | ||||
| P1125 (1) | APP System | ||||
| P1191 (1) | Intake Air Duct Leak | ||||
| P1214 (1) | Injection Pump Timing Offset Error | ||||
| P1216 (1) | Fuel Solenoid Response Time Too Short | ||||
| P1217 (1) | Fuel Solenoid Response Time Too Long | ||||
| P1218 (1) | Injection Pump Calibration Circuit | ||||
| P1406 (1) | EGR Valve Position | ||||
| P1409 (1) | EGR Vacuum System Leak | ||||
| P1626 (1) | VTD Controller Serial Data Circuit | ||||
| P1626 (1) | ("C" & "K" Series) | ||||
| P1621 (1) | EEPROM Write | ||||
| P1627 (1) | A/D Performance | ||||
| P1630 (1) | Theft Deterrent PCM In Learn Mode | ||||
| P1630 (1) | ("C" & "K" Series) | ||||
| P1631 (1) | Theft Deterrent Password Incorrect | ||||
| P1631 (1) | ("C" & "K" Series) | ||||
| P1635 (1) | PCM 5-Volt Reference Low | ||||
| P1641 (1) | MIL Control Circuit | ||||
| P1643 (1) | WAIT TO START Light Control Circuit | ||||
| P1643 (1) | ("C" & "K" Series) | ||||
| P1653 (1) | EGR Vent Sol. Control Circuit | ||||
| P1654 (1) | SERVICE THROTTLE SOON Light Control Circuit | ||||
| P1655 (1) | EGR Solenoid Control Circuit | ||||
| P1656 (1) | Wastegate Solenoid Control Circuit | ||||
| P1810 (2) | Fluid Press. Manual Valve Position Switch | ||||
| P1811 (2) | Maximum Adapt Long Shift | ||||
| P1860 (2) | TCC PWM Solenoid Circuit - Electrical | ||||
| P1870 (2) | Transmission Component Slipping | ||||
| P1871 (2) | Undefined Gear Ratio | ||||
| P1875 (2) | 4WD Low Switch - Input Malfunction ("K" Series) | ||||
|
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Factory Service Tools
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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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