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, command AIR pump relay on and off. If relay turns on and off as commanded, go to next step. If relay does not turn on or off when commanded, go to step  5.
  3. Turn ignition off. Disconnect PCM harness connector containing relay control circuit. Turn ignition on. Using a DVOM set to 10-amp scale, check current from relay control circuit in PCM harness connector to ground for 2 minutes. If current draw is 0.10-0.75 amp, see DIAGNOSTIC AIDS . If current draw is not 0.10-0.75 amp, go to next step.
  4. Turn ignition off. Remove AIR pump relay. Check resistance from relay control circuit in PCM harness connector to ground. If resistance is infinite, go to step  11. If resistance is not infinite, go to step  9.
  5. Turn ignition off. Remove AIR pump relay. Connect test light between relay coil terminals in relay harness connector. Turn ignition on. Using scan tool, command relay on and off. If test light turns on and off with each command, go to step  8. If test light does not turn on and off with each command, go to next step.
  6. Connect test light to ground and probe ignition feed circuit in AIR pump relay harness connector. If test light illuminates, go to next step. If test light does not illuminate, go to step  11.
  7. Turn ignition off. Reinstall AIR pump relay. Disconnect PCM harness connector. Turn ignition on. Connect a fused jumper wire between ground and PCM harness connector relay control circuit. If relay operates, go to step  12. If relay does not operate, go to step  9.
  8. Check connections at AIR pump relay. Repair as necessary. After repairs, go to step  14. If relay connection is okay, go to step  11.
  9. Repair relay control circuit. After repairs, go to step  14.
  10. Repair ignition feed circuit to relay coil. After repairs, go to step  14.
  11. Replace relay. After replacing relay, go to step  14.
  12. Check for poor connection at PCM harness connector. Repair as necessary. After repairs, go to step  14. If connections are okay, go to next step.
  13. Replace PCM. Program replacement PCM using required equipment. After replacing PCM, go to next step.
  14. Using scan tool, select DTC, CLEAR INFO function. Start and warm engine to normal operating temperature. Select DTC, SPECIFIC, then enter DTC P0418. Operate vehicle within conditions for setting this DTC. If scan tool indicates that this test ran and passed, go to next step. If scan tool does not indicate that this test ran and passed, repeat step  2.
  15. Using scan tool, select CAPTURE INFO, REVIEW INFO function. If any undiagnosed DTCs are displayed, go to applicable DTC test.
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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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