Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCESYSTEMENGINE CONTROL SYSTEM - TESTS W/CODES - DTC P0135-P0204DTC P0135DTC P0135: HO2S HEATER CIRCUIT-BANK 1, SENSOR 1 (4.8L, 5.3L & 6.0L "C" & "K" SERIES)DIAGNOSTIC PROCEDURES
1999 GMC Forward Control 5.7 R, Automatic
Diagnostic Procedures
1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedures
- 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.
- Ensure engine and HO2S 1 are completely cool. Turn ignition on. Using scan tool, monitor HO2S 1 voltage. If voltage changes from about 450 mV to less than 300 mV or greater than 700 mV, see DIAGNOSTIC AIDS . If voltage does not change as specified, go to next step.
- Check for open in fuse that supplies current to HO2S 1 heater circuit. If fuse is open, go to step 11. If fuse is okay, go to next step.
- Disconnect HO2S 1 harness connector. Using a test light connected to ground, probe HO2S 1 heater ignition feed circuit. If test light illuminates, go to next step. If test light does not illuminate, go to step 7.
- Connect test light between HO2S 1 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 8.
- Measure resistance of HO2S 1 between heater ignition feed and heater ground circuits. If resistance is 3.5-14.0 ohms, go to step 9. If resistance is not as specified, go to step 10.
- Repair open HO2S 1 heater ignition feed circuit. After repairs, go to step 12.
- Repair open HO2S 1 heater ground circuit. After repairs, go to step 12.
- Check for faulty HO2S 1 harness connections. Repair as necessary. After repairs, go to step 12. If connections are okay, go to next step.
- Replace HO2S 1. After repairs, go to step 12.
- Repair short to ground in HO2S 1 heater ignition feed circuit. Replace fuse. After repairs, go to next step.
- Using scan tool, clear DTCs. Start engine and allow it to reach normal operating temperature. 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.
- Using scan tool, select CAPTURE INFO, REVIEW INFO. If any undiagnosed DTCs are displayed, go to applicable DTC test. If not additional 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.