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. After performing OBD system check, go to next step.
  2. Using scan tool, clear DTCs. Reset fuel trim values. Start engine and allow it to reach normal operating temperature. Monitor HO2S 1 voltage on scan tool. If voltage is fixed at 351-551 mV, go to step  4. If voltage is not as specified, go to next step.
  3. DTC is intermittent. Faulty wiring and harness connections or exhaust leaks may have set this DTC. Repair as necessary. After repairs, go to step  17. If no problem is found, see DIAGNOSTIC AIDS .
  4. Turn ignition off. Disconnect HO2S 1 harness connector. Using jumper wires, jumper HO2S 1 high and low signal circuits to ground. Turn ignition on, with engine off. Monitor HO2S 1 voltage with scan tool. If voltage is less than 20 mV, go to step  7. If voltage is not less than 20 mV, go to next step.
  5. Turn ignition off. Remove jumper wires. Disconnect PCM harness connector containing HO2S 1 circuits. Measure resistance of HO2S 1 high and low signal circuits. If each circuit resistance is less than 5 ohms, go to next step. If either circuit resistance is not less than 5 ohms, go to step  9.
  6. Check for faulty PCM harness connections. Repair as necessary. After repairs, go to step  17. If connections are okay, go to step  16.
  7. Remove jumper wires. Turn ignition on, with engine off. Connect a test light between heater ignition feed and heater ground circuits at HO2S 1 harness connector. If test light illuminates, go to next step. If test light does not illuminate, go to step  10.
  8. Turn ignition off. Reconnect HO2S 1 harness connector. Disconnect PCM harness connector containing HO2S 1 circuits. Measure resistance between HO2S 1 low signal circuit and any PCM ground circuit at PCM harness connector. If resistance is less than 500 ohms, go to step  12. If resistance is not less than 500 ohms, go to step  11.
  9. Repair circuit with high resistance. After repairs, go to step  17.
  10. Repair HO2S 1 heater ignition feed or ground circuit. After repairs, go to step  17.
  11. Repair high resistance between HO2S 1 low circuit and PCM ground circuit. After repairs, go to step  17.
  12. Check for an exhaust leak upstream of HO2S 1. Repair as necessary. After repairs, go to step  17. If no leaks are found, go to next step.
  13. Allow engine to completely cool. Measure resistance between HO2S 1 low signal circuit and any PCM ground circuit at PCM harness connector. If resistance is less than 500 ohms, go to step  15. If resistance is not less than 500 ohms, go to next step.
  14. Remove HO2S 1. Clean threads and apply anti-seize compound, and then reinstall. Measure resistance between HO2S 1 low signal circuit and any PCM ground circuit at PCM harness connector. If resistance is less than 500 ohms, go to step  17. If resistance is not less than 500 ohms, go to next step.
  15. Replace HO2S 1. After repairs, go to step  17.
  16. Replace PCM. Program replacement PCM using required equipment. After repairs, go to next step.
  17. Using scan tool, clear DTCs. Turn ignition off and wait 15 seconds. Start engine and operate vehicle under conditions required to set this DTC. If scan tool indicates this test ran and passed, go to next step. If scan tool does not indicate this test ran and passed, go to step  2.
  18. Using scan tool, select CAPTURE INFO, REVIEW INFO function. If any undiagnosed DTCs are displayed, go to applicable DTC test. If no undiagnosed DTCs are displayed, system is okay.
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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