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HOMESERVICE MANUALSCHEVROLET2003TRAILBLAZER 4.2 S, 4WDREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 218 (ENGINE CONTROL SYSTEM - SELF-DIAGNOSTICS)DIAGNOSTIC TESTSDTC P0121: TP SENSOR PERFORMANCEDIAGNOSTIC PROCEDURES
2003 Chevrolet TrailBlazer 4.2 S, 4WD

Diagnostic Procedures

2003 Chevrolet TrailBlazer 4.2 S, 4WDSECTION Diagnostic Procedures
WARNING: This page is about a different car, the 2003 Pontiac Vibe. However, it is still accessible from the selected car via links, so may be relevant.
  1. Did you perform the Diagnostic System Check - Engine Controls? If yes, go to next step. If no, perform diagnostic system check - engine controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM.
  2. Turn ignition switch to RUN position, engine OFF. Install scan tool. Select TP ANGLE parameter. Monitor scan tool while depressing accelerator pedal, repeat procedure several times. Does TP angle value increase steadily when accelerator pedal is depressed and decrease steadily when pedal is released, within specified range? If yes, go to next step. If no, go to step  4.
  3. Use scan tool to clear DTCs. Place vehicle into CHECK MODE operation, using scan tool. See SERVICE BAY TESTS  under SELF-DIAGNOSTIC SYSTEM. Operate vehicle within freeze frame conditions as specified. Does DTC P0121 set? If yes, go to next step. If no, see DIAGNOSTIC AIDS .
  4. Connect DVOM between TP sensor reference voltage circuit on harness side and ground. Turn ignition switch to RUN position, engine OFF. Is TP sensor voltage at zero volts? If yes, go to next step. If no, go to step  6.
  5. Connect a test light between reference voltage circuit and ground. Turn ignition switch to RUN position, engine OFF. Does test light illuminate at least dimly? If yes, go to step  7. If no, go to step  9.
  6. Check for the following conditions:
    • A short to voltage in TP sensor input circuit.
    • Proper mating of TP sensor harness connectors.
    Repair as necessary. After repair, go to step  12.
  7. Connect a test light between TP sensor input circuit and TP sensor reference voltage circuit at harness connector. Monitor TP angle on scan tool. Is TP sensor percentage more than 90 percent? If yes, go to step  10. If no, go to next step.
  8. Check for the following conditions:
    • High resistance in TP sensor ground circuit between sensor and splice S206.
    • High resistance in TP sensor input circuit between TP sensor and PCM.
    Repair as necessary. Was a repair necessary? If yes, go to step  12. If no, go to step  11.
  9. Locate and repair high resistance in reference voltage circuit of TP sensor. After repair, go to step  12.
  10. Replace TP sensor. After repair, go to step  12.
  11. Replace PCM. After repair, go to next step.
  12. Use scan tool to 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.
  13. With a scan tool, observe stored information, CAPTURE INFO. Does scan tool display any DTCs that you have not diagnosed? If yes, go to DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, system is okay at this time.
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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.