1999 GMC Forward Control 5.7 R, AutomaticFACTORY OEM SERVICE PROCEDURE

Diagnostic Procedures

  1. Perform On-Board Diagnostic (OBD) system check. See ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS INTRODUCTION article. After performing OBD system check, go to next step.
  2. Turn ignition on, with engine off. Using scan tool, monitor ECT sensor temperature. If ECT sensor temperature is greater than 282°F (139°C), go to step  6. If ECT sensor temperature is not greater than 282°F (139°C), go to next step.
  3. If ECT sensor temperature is less than -45°F (-43°C), go to step  8. If ECT sensor temperature is -45°F (-43°C) or greater, go to next step.
  4. Turn ignition on, with engine off. Move/wiggle ECT sensor and ECM wiring harness and connectors while observing scan tool display. If ECT sensor value on scan tool display changes when moving wiring harness or connectors, go to step  12. If ECT sensor value on scan tool display does not change when moving wiring harness or connectors, go to next step.
  5. Turn ignition on, with engine off. Review FREEZE FRAME data for this DTC and note parameters. Clear DTCs. Turn ignition off for 15 seconds. Start engine and operate vehicle as close to conditions recorded in FREEZE FRAME as possible. Check for DTCs. If DTC P0115 is set, go to next step. If DTC P0115 is not set, see DIAGNOSTIC AIDS .
  6. Disconnect ECT sensor connector. Observe scan tool display. If ECT sensor temperature is less than -45°F (-43°C), go to step  17. If ECT sensor temperature is -45°F (-43°C) or greater, go to next step.
  7. Turn ignition off. Disconnect ECM harness connector located in engine compartment relay center. Using DVOM, check for continuity between ECT sensor signal circuit and ground. If continuity is present, go to step  13. If continuity is not present, go to step  19.
  8. Disconnect ECT sensor connector. Jumper ECT sensor harness connector terminals together. Observe scan tool display. If ECT sensor temperature is greater than 282°F (139°C), go to step  16. If ECT sensor temperature is not greater than 282°F (139°C), go to next step.
  9. Jumper ECT sensor signal circuit to a known-good ground. Observe scan tool display. If ECT sensor temperature is greater than 282°F (139°C), go to step  11. If ECT sensor temperature is not greater than 282°F (139°C), go to next step.
  10. Turn ignition off. Disconnect ECM harness connector located in engine compartment relay center. Using DVOM, check continuity of sensor signal circuit between ECM and ECT sensor harness connectors. If continuity is present, go to step  18. If continuity is not present, go to step  14.
  11. Turn ignition off. Disconnect ECM harness connector located in engine compartment relay center. Using DVOM, check continuity of sensor ground circuit between ECM and ECT sensor harness connectors. If continuity is present, go to step  18. If continuity is not present, go to step  15.
  12. Repair harness or connections as necessary. After repairs, go to step  20.
  13. Repair short to ground in signal circuit. After repairs, go to step  20.
  14. Repair open or high resistance in signal circuit. After repairs, go to step  20.
  15. Repair open or high resistance in ground circuit. After repairs, go to step  20.
  16. Check for poor connections and terminal tension at ECT sensor harness connector. Repair as necessary. After repairs, go to step  20. If connections and terminals are okay, go to next step.
  17. Replace ECT sensor. After replacing sensor, go to step  20.
  18. Check for poor connections and terminal tension at ECM harness connector. Repair as necessary. After repairs, go to step  20. If connections and terminals are okay, go to next step.
  19. Replace ECM. New ECM must be programmed with theft deterrent module frequency code currently on vehicle. After repairs, go to next step.
  20. Using scan tool, clear DTCs. Turn ignition off for 15 seconds. Start engine and operate vehicle as close to conditions recorded in FREEZE FRAME as possible. Check for DTCs. If DTC P0115 is set, go to step  2. If DTC P0115 is not set, go to next step.
  21. If other DTCs were present during OBD system check and have not been diagnosed, go to applicable DTC. If no other DTCs are present, system is okay.
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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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