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Home >> Buick >> 2003 >> Century Custom >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls Self-Diagnostics - 3.1L Engine >> Diagnostic Tests >> DTC P0121: TP Sensor Performance >> Diagnostic Procedure
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Diagnostic Procedure

  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. Check for the following conditions:
    • Any vacuum hoses disconnected, damaged, or incorrectly routed.
    • The Manifold Absolute Pressure (MAP) sensor vacuum hose disconnected or damaged.
    • Restrictions in the MAP sensor vacuum source.
    • Any intake manifold vacuum leaks.

    Did you find and correct the condition? If yes, go to step  24. If no, go to next step.

    3. NOTE: The vehicle used for the comparison is not limited to the same type of vehicle as is being serviced. A vehicle known to provide an accurate reading is acceptable.
  3. Do you have access to another vehicle in which the MAP sensor pressure can be observed using a scan tool? If yes, go to next step. If no, go to step  5.
  4. Turn ignition switch to RUN position, engine OFF. Observe the MAP sensor pressure using a scan tool. Observe the MAP sensor pressure in the known good vehicle using a scan tool. Compare the values. Is the difference between the values less than 3 kPa? If yes, go to step  6. If no, go to MANIFOLD ABSOLUTE PRESSURE SENSOR under SENSORS & SWITCHES in SYSTEM & COMPONENT TESTS - 3.1L CENTURY, GRAND PRIX & MALIBU article.
    5. NOTE: Altitude vs Barometric Pressure table indicates a pressure range for a given altitude under normal weather conditions. Weather conditions consisting of very low or very high pressure and/or very low or very high temperature may cause a reading to be slightly out of range.
  5. Turn ignition switch to RUN position, engine OFF. Observe the MAP sensor pressure using a scan tool. See ALTITUDE VS. BAROMETRIC PRESSURE  table. The MAP sensor pressure should be within the range specified for your altitude. Does the MAP sensor indicate the correct barometric pressure? If yes, go to next step. If no, go to MANIFOLD ABSOLUTE PRESSURE SENSOR under SENSORS & SWITCHES in SYSTEM & COMPONENT TESTS - 3.1L CENTURY, GRAND PRIX & MALIBU article.
  6. Observe the MAP sensor pressure using a scan tool. Start the engine. Does the MAP sensor pressure change? If yes, go to next step. If no, go to MANIFOLD ABSOLUTE PRESSURE SENSOR under SENSORS & SWITCHES in SYSTEM & COMPONENT TESTS - 3.1L CENTURY, GRAND PRIX & MALIBU article.
  7. Turn ignition switch to OFF position. Turn ignition switch to RUN position, engine OFF. Observe the Throttle Position (TP) sensor parameter using the scan tool. Is the voltage less than 0.90 volt with the throttle in the closed position? If yes, go to next step. If no, go to step  9.
  8. Observe the TP sensor parameter using the scan tool. Depress the accelerator pedal slowly until the throttle is in the wide open position. Release the accelerator pedal slowly until the throttle is returned to the closed position. Repeat this procedure several times. Does the TP sensor parameter increase steadily to more than 98 percent, and then decrease steadily returning to less than one percent? If yes, go to INTERMITTENTS in TROUBLE SHOOTING - 3.1L CENTURY, GRAND PRIX & MALIBU article. If no, go to step  10.
  9. Check the throttle body for the following conditions:
    • A binding or damaged throttle blade.
    • Binding or damaged throttle cable.

    Did you find and correct the condition? If yes, go to step  24. If no, go to next step.

  10. Disconnect the TP sensor harness connector. Observe the TP sensor parameter using the scan tool. Is the voltage less than 0.1 volt? If yes, go to next step. If no, go to step  15.
  11. Measure the voltage from the TP sensor 5-volt reference circuit to a good ground using a DMM. Is the voltage more than 5.2 volts? If yes, go to step  16. If no, go to next step.
  12. Probe the TP sensor 5-volt reference circuit using a test light connected to a good ground. Is the test light OFF? If yes, go to step  17. If no, go to next step.
  13. Connect a 3-amp fused jumper wire between the TP sensor 5-volt reference circuit and the TP sensor signal circuit. Observe the TP sensor parameter using the scan tool. Is the voltage more than 4.9 volts? If yes, go to next step. If no, go to step  18.
  14. Turn ignition switch to OFF position. Remove the jumper wire. Connect a jumper wire between each of the terminals in the TP sensor harness connector and the corresponding terminal at the TP sensor. Turn ignition switch to RUN position, engine OFF. Measure the voltage from the TP sensor low reference circuit at the jumper wire terminal to a good ground using a DMM. Is the voltage more than 0.2 volt? If yes, go to step  19. If no, go to step  20.
    15. NOTE: Disconnecting the Powertrain Control Module (PCM) may eliminate the short during testing.
  15. Test the TP sensor signal circuit between the PCM and the TP sensor for a short to voltage. Did you find and correct the condition? If yes, go to step  24. If no, go to step  23.
    16. NOTE: Disconnecting the Powertrain Control Module (PCM) may eliminate the short during testing.
  16. Test all of the branches of the 5-volt reference circuit that are shared with the TP sensor for a short to voltage. Did you find and correct the condition? If yes, go to step  24. If no, go to step  23.
  17. Test the 5-volt reference circuit between the PCM and the TP sensor for an open or for a high resistance. Did you find and correct the condition? If yes, go to step  24. If no, go to step  21.
  18. Test the TP sensor signal circuit between the PCM and the TP sensor for the following conditions:
    • A short to ground.
    • An open.
    • A high resistance.

    Did you find and correct the condition? If yes, go to step  24. If no, go to step  21.

  19. Test the low reference circuit between the PCM and the TP sensor for an open or for a high resistance. Did you find and correct the condition? If yes, go to step  24. If no, go to step  21.
  20. Test for an intermittent and for a poor connection at the TP sensor. Did you find and correct the condition? If yes, go to step  24. If no, go to step  22.
  21. Test for an intermittent and for a poor connection at the PCM. Did you find and correct the condition? If yes, go to step  24. If no, go to step  23.
  22. Replace the TP sensor. After repair, go to step  24.
  23. Replace PCM. See POWERTRAIN CONTROL MODULE in appropriate REMOVAL & INSTALLATION article. Perform PCM relearn procedure. See POWERTRAIN CONTROL MODULE  under PROGRAMMING. After replacing PCM, go to next step.
  24. Clear the DTCs using a scan tool. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running Test. See CONDITIONS FOR RUNNING TEST . You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records. Did the DTC fail this ignition? If yes, go to step  2. If no, go to next step.
  25. Using the scan tool, observe the stored information in 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 at this time.
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