Introduction
The P0120 diagnostic trouble code indicates a general circuit failure in the Throttle Position Sensor (TPS) “A” or Pedal Position Sensor circuit. The TPS measures how far the throttle plate is open and feeds this data directly to the powertrain control module (PCM) to control fuel delivery and ignition timing. If the PCM receives erratic, out-of-range, or missing voltage readings from Sensor A, it sets code P0120 and often enters a protective limp mode.
Common Symptoms
- Check Engine Light illuminated on the instrument cluster.
- Vehicle entering “Limp Mode” with severely reduced engine power.
- Unresponsive throttle pedal or sudden power surges while driving.
- Rough, uneven idling or engine stalling.
Primary Causes
- Faulty Throttle Position Sensor: Worn internal potentiometer tracks or electrical failure inside sensor “A”.
- Corroded or Loose Connector: Poor pin contact or water intrusion at the TPS plug.
- Damaged Wiring Harness: Shorted, broken, or rubbed-through signal wires in the TPS harness.
- Faulty Electronic Throttle Body: Internal motor failure or worn position tracks in integrated electronic throttle bodies.
Step-by-Step Diagnosis & Fixes
- Inspect Connector and Wiring: Inspect the harness plug connected to the throttle body for bent pins, corrosion, or damaged wires.
- Check Reference Voltage: Use a multimeter to confirm 5V reference voltage and ground supply reaching the TPS plug.
- Monitor Signal Voltage: Sweep the throttle slowly while measuring signal voltage with a multimeter or scanner to verify smooth voltage change without dead spots.
- Replace Sensor or Throttle Assembly: Replace the standalone TPS sensor (or the complete electronic throttle assembly if the sensor is integrated).