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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 78 (ENGINE CONTROL SYSTEM SELF-DIAGNOSTICS - 3.8L)DIAGNOSTIC TESTSDTC P0135: HO2S HEATER PERFORMANCE - SENSOR 1DIAGNOSTIC PROCEDURES
1999 GMC Forward Control 5.7 R, Automatic

Diagnostic Procedures

1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedures
WARNING: This page does not describe the selected car, but rather 7 other vehicles, including the 2002 Pontiac Grand Prix, 2002 Pontiac Bonneville, 2002 Chevrolet Monte Carlo, 2002 Chevrolet Impala, and 2002 Buick Regal. However, it is still accessible from the selected car via links, so may be relevant.
  1. Perform Diagnostic System Check - Engine Controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM. After performing Diagnostic System Check - Engine Controls, go to next step.
    2. NOTE: If the engine has just been operating, allow engine to cool until near bias voltage is observed on the scan tool before proceeding.
  2. Turn on the ignition, with the engine off. Monitor HO2S 1 voltage on the scan tool. Does the HO2S 1 voltage go to above or below the previously stabilized voltage, by at least as much as 100 millivolts within 2 minutes? If yes, go to next step. If no, go to step  4.
  3. Observe the Freeze Frame/Failure Records data for this DTC. Turn off the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions for Running DTC or as close to the Freeze Frame/Failure Records data that you observed. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see INTERMITTENT CONDITIONS  under SELF-DIAGNOSTIC SYSTEM.
  4. Disconnect the HO2S 1 electrical connector. Turn on the ignition, with the engine off. Connect a test lamp between the HO2S 1 heater ignition voltage circuit, in the harness side of the HO2S connector, and ground. Does the test lamp illuminate? If yes, go to next step. If no, go to step  7.
  5. Turn off the ignition. Connect a test lamp between the HO2S 2 heater ignition voltage circuit, in the harness side of the HO2S connector, and the HO2S 2 heater ground circuit. Turn on the ignition, with the engine off. Does the test lamp illuminate? If yes, go to next step. If no, go to step  10.
    6. NOTE: The heater element resistance will vary according to HO2S temperature. A hot HO2S heater element will have a higher resistance than a heater element at room temperature.
  6. Turn off the ignition. Leave the HO2S 1 connector disconnected. Allow the engine to cool for at least 15 minutes. Measure the HO2S 1 heater resistance at the HO2S 1 pigtail connector using a DMM. Does the resistance measure 2-20 ohms? If yes, go to step  11. If no, go to step  12.
  7. Test the HO2S 1 heater ignition 1 voltage circuit for an open fuse. Did you find an open fuse? If yes, go to next step. If no, go to step  9.
  8. Locate and repair the short to ground in the HO2S 1 ignition 1 circuit and replace the fuse. After repairs, go to step  13.
  9. Repair the open or high resistance condition in the HO2S 1 ignition 1 circuit. After repairs, go to step  13.
  10. Repair the open or high resistance condition in the HO2S 1 heater ground circuit. After repairs, go to step  13.
  11. Before continuing, inspect for poor connections at the HO2S 1 connector. Did you find and correct the condition? If yes, go to step  13. If no, go to next step.
  12. Replace HO2S. After repairs, go to step  13.
  13. Using scan tool, clear DTCs. Turn ignition off for 30 seconds. Start engine. Operate vehicle within Conditions for Running DTC. Does DTC run and pass? If yes, go to next step. If no, go to step  2.
  14. Using scan tool, observe stored information. Does scan tool displays any DTCs that have not been diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, 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.