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. Are DTCs P0412 or P0418 set? If yes, diagnose affected DTC(s). If no, go to next step.
  3. Is the AIR pump relay switch feed fuse okay? If yes, go to next step. If no, go to step  13.
  4. With a scan tool, command the AIR pump ON and OFF. Does the AIR pump turn ON and OFF with each command? If yes, go to step  10. If no, go to next step.
  5. Does the AIR pump run continuously? If yes, go to step  16. If no, go to next step.
  6. Disconnect the AIR pump relay. Probe the relay switch feed circuit using a Test Light (J 34142-B). Does the test light illuminate? If yes, go to next step. If no, go to step  17.
  7. With a fused jumper wire, jumper the relay switch feed circuit to the AIR pump feed circuit. Does the AIR pump operate? If yes, go to step  25. If no, go to next step.
  8. Leave the fused jumper in place. Disconnect the AIR pump. Probe the feed circuit of the pump with a test light connected to a good ground. Does the test light illuminate? If yes, go to next step. If no, go to step  18.
  9. Connect test light between the feed circuit of the pump and ground circuit of the pump. Does the test light illuminate? If yes, go to step  27. If no, go to step  19.
  10. Disconnect the AIR hose/pipe from the AIR Pump. With scan tool, command the AIR pump ON. Is air flow present at the AIR pump outlet? If yes, go to next step. If no, go to step  28.
  11. Reconnect the AIR hose/pipe to the AIR pump. Disconnect the AIR hose/pipe from the AIR combination valve. With scan tool, command the AIR pump ON. Is air flow present at the AIR hose/pipe outlet? If yes, go to next step. If no, go to step  22.
  12. Start engine. Visually inspect for engine vacuum at the AIR combination valve. If vacuum is not present, inspect for a damaged, pinched or disconnected vacuum line, a blocked/restricted vacuum port or a faulty vacuum control solenoid valve. Inspect for a damaged, pinched or disconnected vacuum line between the vacuum control solenoid and actuator diaphragm. Was a problem found? If yes, go to step  23. If no, go to step  26.
  13. Disconnect AIR pump relay. Test feed circuit of the relay switch for a short to ground. Was a problem found? If yes, go to step  20. If no, go to next step.
  14. Disconnect AIR pump. Test the feed circuit of the pump for a short to ground. Was a problem found? If yes, go to step  21. If no, go to next step.
  15. Reconnect relay. Reconnect AIR pump. Install a new fuse. With scan tool, command the AIR pump ON. Does the fuse open? If yes, go to step  28. If no, see DIAGNOSTIC AIDS .
  16. Disconnect AIR pump relay. Is pump still running? If yes, go to step  24. If no, go to step  25.
  17. Repair open relay switch feed circuit. After repairs, go to step  29.
  18. Test for an open in the AIR pump feed circuit. Repair circuit as necessary. Did you find and correct the condition? If yes, go to step  29. If no, go to step 25.
  19. Repair open/high resistance in the AIR pump ground circuit. After repairs, go to step  29.
  20. Repair short to ground in the relay switch feed circuit. After repairs, go to  29.
  21. Repair short to ground in the AIR pump feed circuit. After repairs, go to step  29.
  22. Visually inspect and repair as necessary for a blocked or damaged combination valve, a restriction, blockage, disconnect or other damage to the AIR hoses/pipes from the combination valve to the exhaust system. After repairs, go to step  29.
  23. Repair vacuum system as necessary. After repairs, go to step  29.
  24. Repair short to voltage in the AIR pump feed circuit. After repairs, go to step  29.
  25. Replace AIR pump relay. After replacing relay, go to step  29.
  26. Replace AIR combination valve. After replacing valve, go to step  29.
  27. Check for poor connections at the AIR pump. Repair as necessary. Did you find and correct the condition? If yes, go to step  29. If no, go to next step.
  28. Replace AIR pump. After replacing AIR pump, go to next step.
  29. Using scan tool, clear the DTCs. Turn OFF the ignition and wait 15 seconds. Operate vehicle within the conditions that are required for this diagnostic to run. Refer to CONDITIONS FOR RUNNING DTC . Does scan tool indicate that this test ran and passed? If yes, go to next step. If no, go to step  2.
  30. Using scan tool, select CAPTURED INFO function. If any undiagnosed DTCs are displayed, go to applicable DTC test. If no DTCs are displayed, 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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