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DTC P0121: Throttle Position Sensor #1 Performance: Testing

NOTE: Diagnose any other TP sensor DTCs first before continuing. See DIAGNOSTIC TROUBLE CODE DEFINITIONS . The throttle plate and linkage should be free of binding and carbon build-up. Ensure the throttle plate is at the idle position.
  1. Ignition on, engine not running. With the DRBIII® scan tool, read DTCs and record the related freeze frame data. Is the Good Trip Counter displayed and equal to zero? If yes, go to next step. If no, see INTERMITTENT CONDITION  under SELF-DIAGNOSTIC SYSTEM.
  2. Start the engine. With the DRBIII® scan tool monitor the MAP sensor voltage. Snap the throttle. Does the DRBIII® scan tool display MAP voltage from below 2.0 volts at idle to above 3.5 volts at WOT? If yes, go to next step. If no, go to step  11.
  3. Ignition on, engine not running. With the DRBIII® scan tool, monitor the TP sensor voltage while slowly depressing the throttle pedal from the idle position to the wide open throttle position. Does voltage start at approximately 0.8 volt and go above 3.5 volts with a smooth transition? If yes, see INTERMITTENT CONDITION  under SELF-DIAGNOSTIC SYSTEM. If no, go to next step.
    4. CAUTION: Do not probe the PCM harness connectors. Probing the PCM harness connectors will damage the PCM terminals resulting in poor terminal to pin connection. Install Miller Special Tool (8815) to perform diagnosis.
  4. Turn the ignition off. Disconnect the TP sensor harness connector. Disconnect the PCM harness connectors. Measure the resistance of the (K6) 5-volt supply circuit from the TP sensor harness connector to the appropriate terminal of Miller Special Tool (8815). Is the resistance below 5.0 ohms? If yes, go to next step. If no, repair the (K6) 5-volt supply circuit for an open circuit.
  5. Turn the ignition off. Disconnect the TP sensor harness connector. Disconnect the PCM harness connectors. Measure the resistance between ground and the (K6) 5-volt supply circuit at the TP sensor harness connector. Is the resistance above 100 k/ohms? If yes, go to next step. If no, repair the (K6) 5-volt supply circuit.
  6. Ensure ignition is off. Disconnect the TP harness connector. Reconnect the PCM harness connectors. With the DRBIII® scan tool, monitor the TP sensor voltage. Turn the ignition on, engine not running. Connect a jumper wire between the (K22) TP signal circuit and the (K4) sensor ground circuit. Does the TP sensor voltage change from approximately 4.9 volts to below 0.5 volt? If yes, replace the TP sensor. If no, go to next step.
    7. CAUTION: Do not probe the PCM harness connectors. Probing the PCM harness connectors will damage the PCM terminals resulting in poor terminal to pin connection. Install Miller Special Tool (8815) to perform diagnosis.
  7. Turn the ignition off. Disconnect the TP harness connector. Disconnect the PCM harness connectors. Measure the resistance of the (K22) TP signal circuit from the TP sensor harness connector to the appropriate terminal of Miller Special Tool (8815). Is the resistance below 5.0 ohms? If yes, go to next step. If no, repair the TP signal circuit.
  8. Turn the ignition off. Disconnect the TP harness connector. Disconnect the PCM harness connectors. Measure the resistance between ground and the (K22) TP signal circuit in the TP sensor harness connector. Is the resistance above 100 k/ohms? If yes, go to next step. If no, repair the TP (K22) sensor signal circuit.
    9. CAUTION: Do not probe the PCM harness connectors. Probing the PCM harness connectors will damage the PCM terminals resulting in poor terminal to pin connection. Install Miller Special Tool (8815) to perform diagnosis.
  9. Turn the ignition off. Disconnect the TP harness connector. Disconnect the PCM harness connectors. Measure the resistance of the (K4) sensor ground circuit from the TP sensor harness connector to the appropriate terminal of Miller Special Tool (8815). Is the resistance below 5.0 ohms? If yes, go to next step. If no, repair the (K4) sensor ground circuit.
    10. NOTE: Before continuing, disconnect the PCM harness connectors and check the related wiring terminals for corrosion, damage or terminal push out. Repair as necessary.
  10. There are no possible causes remaining, replace PCM. Program the new PCM. See PROGRAMMING .
    11. CAUTION: Do not probe the PCM harness connectors. Probing the PCM harness connectors will damage the PCM terminals resulting in poor terminal to pin connection. Install Miller Special Tool (8815) to perform diagnosis.
  11. Turn the ignition off. Disconnect the MAP sensor harness connector. Disconnect the PCM harness connectors. Measure the resistance of the (K6) 5-volt supply circuit from the MAP sensor harness connector to the appropriate terminal of Miller Special Tool (8815). Is the resistance below 5.0 ohms? If yes, go to next step. If no, repair the MAP (K6) 5-volt supply circuit.
  12. Turn the ignition off. Disconnect the MAP harness connector. Disconnect the PCM harness connectors. Measure the resistance between ground and the (K6) 5-volt supply circuit at the MAP sensor harness connector. Is the resistance above 100 k/ohms? If yes, go to next step. If no, repair the (K6) 5-volt supply circuit for a short to ground.
  13. Turn the ignition off. Disconnect the MAP harness connector. Reconnect the PCM harness connectors. With the DRBIII® scan tool, monitor the MAP sensor voltage. Ignition on, engine not running. Connect a jumper wire between the (K1) MAP sensor signal circuit and the (K4) sensor ground circuit. Cycle the ignition switch from off to on. With the DRBIII® scan tool, monitor the MAP sensor voltage. Does the DRBIII® scan tool display MAP voltage from approximately 4.9 volts to below 0.5 volt? If yes, replace the MAP sensor. If no, go to next step.
    14. CAUTION: Do not probe the PCM harness connectors. Probing the PCM harness connectors will damage the PCM terminals resulting in poor terminal to pin connection. Install Miller Special Tool (8815) to perform diagnosis.
  14. Turn the ignition off. Disconnect the MAP sensor harness connectors. Disconnect the PCM harness connectors. Measure the resistance of the (K1) MAP signal circuit from the MAP sensor harness connector to the appropriate terminal of Miller Special Tool (8815). Is the resistance below 5.0 ohms? If yes, go to next step. If no, repair the (K1) MAP signal circuit.
  15. Turn the ignition off. Disconnect the MAP sensor harness connectors. Disconnect the PCM harness connectors. Measure the resistance between ground and the (K1) MAP signal circuit at the MAP sensor harness connector. Is the resistance above 100 k/ohms? If yes, go to next step. If no, repair the MAP signal circuit.
    16. CAUTION: Do not probe the PCM harness connectors. Probing the PCM harness connectors will damage the PCM terminals resulting in poor terminal to pin connection. Install Miller Special Tool (8815) to perform diagnosis.
  16. Turn the ignition off. Disconnect the MAP sensor harness connector. Disconnect the PCM harness connectors. Measure the resistance of the (K4) sensor ground circuit from the MAP sensor harness connector to the appropriate terminal of Miller Special Tool (8815). Is the resistance below 5.0 ohms? If yes, go to next step. If no, repair the (K4) sensor ground circuit.
    17. NOTE: Before continuing, disconnect the PCM harness connectors and check the related wiring terminals for corrosion, damage or terminal push out. Repair as necessary.
  17. There are no possible causes remaining, replace PCM. Program the new PCM. See PROGRAMMING .
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