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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. Turn ignition on, engine not running. With the DRBIII®, read DTCs and record related Freeze Frame data. Is the Good Trip counter displayed and equal to zero? If yes, go to next step. If no, go to step  10.
  2. Turn ignition on, engine not running. With the DRBIII®, 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 3.6 volts with a smooth transition? If yes, go to step  11. If no, go to next step.
    3. 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. Use Miller Special Tool (8815) when performing diagnosis at PCM harness connectors.
  3. Turn the ignition off. Disconnect the TP Sensor harness connector. Disconnect the PCM harness connectors. Measure the resistance of the 5-Volt Supply circuit from the TP Sensor harness connector to the appropriate terminal of the Miller Special Tool. Is the resistance below 5.0 ohms? If yes, go to next step. If no, repair the high resistance in the TP Sensor 5-Volt Supply circuit.
  4. Measure the resistance between ground and the 5-Volt Supply circuit at the TP Sensor harness connector. Is the resistance above 1.0 ohm? If yes, go to next step. If no, repair the high resistance to ground in the 5-Volt Supply circuit.
  5. Ensure ignition is off. Reconnect PCM harness connectors. With the DRBIII®, monitor the TP Sensor voltage. Turn ignition on, engine not running. Connect a jumper wire between the TP Sensor Signal circuit and the Sensor Ground circuit at the TP Sensor harness connector. Does the DRBIII® display Throttle Position Sensor voltage from approximately 4.9 volts to 0.5 volt? If yes, replace the TP Sensor. If no, go to next step.
    6. 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. Use Miller Special Tool (8815) when performing diagnosis at PCM harness connectors.
  6. Turn the ignition off. Disconnect the PCM harness connectors. Measure the resistance of the TP Sensor Signal circuit from the TP Sensor harness connector to the appropriate terminal of the Miller Special Tool. Is the resistance below 5.0 ohms? If yes, go to next step. If no, repair the high resistance in the TP Sensor Signal circuit.
  7. Measure the resistance between ground and the TP Sensor Signal circuit at the TP Sensor harness connector. Is the resistance above 100 ohms? If yes, go to next step. If no, repair the high resistance to ground in the TP Sensor Signal circuit.
  8. Measure the resistance of the Sensor Ground circuit from the TP Sensor harness connector to the appropriate terminal of the Miller Special Tool. Is the resistance below 5.0 ohms? If yes, go to next step. If no, repair the high resistance in the Sensor Ground circuit.
    9. NOTE: Before continuing, check the PCM harness connectors for corrosion, damage or terminal push out. Repair as necessary.
  9. There are no possible causes remaining, replace PCM. Program the new PCM. See PROGRAMMING .
  10. Turn ignition on, engine not running. With the DRBIII®, monitor the TP Sensor voltage. Slowly open the throttle 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, go to next step. If no, replace the TP Sensor.
    11. WARNING: When the engine is operating, do not stand in a direct line with the fan. Do not put your hands near the pulleys, belts or fan. Do not wear loose clothing.
  11. The conditions that set the DTC are not present at this time. The following may help in identifying the intermittent condition. With the engine running at normal operating temperature, monitor the DRBIII® parameters related to the DTC while wiggling the wiring harness. Look for parameter values to change and/or a DTC to set. Review the DRBIII® Freeze Frame information. If possible, try to duplicate the conditions under which the DTC was set. Refer to any Technical Service Bulletins (TSB) that may apply. Visually inspect the related wiring harness. Look for any chafed, pierced, pinched, or partially broken wires. Visually inspect the related wiring harness connectors. Look for broken, bent, pushed out, or corroded terminals. Were any of the above conditions present? If yes, repair as necessary. If no, test is complete.
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