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HOMESERVICE MANUALSCHEVROLET2003TRAILBLAZER 4.2 S, 4WDREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 89 (ENGINE CONTROL SYSTEM SELF-DIAGNOSTICS - 3.1L ENGINE)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 2002 Pontiac Grand Prix, 2002 Chevrolet Malibu, and 2002 Buick Century. 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.
    NOTE: If any other DTCs are set, diagnose affected DTCs first. See DIAGNOSTIC TROUBLE CODE DEFINITIONS .
  2. Turn ignition on, with engine off. Observe the Throttle Position (TP) sensor parameter on the scan tool while slowly opening the throttle to Wide Open Throttle (WOT). Closed throttle value is 0 percent. WOT value is 98 percent. Does the TP sensor value increase steadily and evenly from the closed throttle value to more than the WOT value? If yes, go to next step. If no, go to step  4.
  3. Observe the Freeze Frame/Failure Records data for this DTC. Turn ignition off for 30 seconds. Operate the vehicle within the Conditions for Running the DTC or as close to the Freeze Frame/Failure Records data that you observed. See CONDITIONS FOR RUNNING DTC . Does the DTC fail this ignition? If yes, go to next step. If no, see DIAGNOSTIC AIDS .
  4. Turn ignition off. Disconnect the TP sensor harness connector. Turn ignition on, with engine off. Observe the TP sensor voltage parameter with a scan tool. Does the scan tool indicate TP sensor voltage at 0 volts? If yes, go to next step. If no, go to step  6.
  5. Turn ignition off. Connect a fused jumper wire between the 5-volt reference circuit and the signal circuit at the TP sensor harness connector. Turn ignition on, with engine off. Observe the TP sensor voltage parameter with a scan tool. Does the scan tool indicate TP sensor voltage at 5 volts? If yes, go to step  9. If no, go to step  7.
  6. Test the TP sensor signal circuit for a short to voltage with the DMM. Did you find and correct the condition? If yes, go to step  15. If no, go to step  12.
  7. Test the TP sensor 5-volt reference circuit for an open or high resistance with the DMM. Did you find and correct the condition? If yes, go to step  15. If no, go to next step.
  8. Test the TP sensor signal circuit for an open or high resistance. Did you find and correct the condition? If yes, go to step  15. If no, go to step  11.
  9. Test the TP sensor low reference circuit for an open or high resistance. Did you find and correct the condition? If yes,  15. If no, go to next step.
  10. Test the TP sensor 5-volt reference circuit for high resistance. Did you find and correct the condition? If yes, go to step  15. If no, go to next step.
  11. Inspect for a poor connection at the TP sensor harness connector. Did you find and correct the condition? If yes, go to step  15. If no, go to step  13.
  12. Inspect for a poor connection at the Powertrain Control Module (PCM) harness connector. Did you find and correct the condition? If yes, go to step  15. If no, go to step  14.
  13. Replace the TP sensor. After replacing sensor, go to step  15.
  14. Replace PCM. Perform PCM relearn procedure. See POWERTRAIN CONTROL MODULE  under PROGRAMMING. After replacing PCM, go to next step.
  15. Clear the DTCs with a scan tool. Turn ignition off for 30 seconds. Start engine. Operate the vehicle within the Conditions for Running the DTC. See CONDITIONS FOR RUNNING DTC . Does the DTC run and pass? If yes, go to next step. If no, go to step  2.
  16. Using scan tool, observe the stored information and Capture Info. Does the scan tool display any DTCs that you have not diagnosed? If yes, diagnose affected DTCs. 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.