P0120 - Throttle/Pedal Position Sensor/Switch 'A' Circuit
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System
Powertrain (Engine & Gearbox)
Related Fault Codes
Technical Description
P0120 occurs when the Engine Control Module (ECM) detects a voltage output from the Throttle Position Sensor (TPS) or Accelerator Pedal Position (APP) sensor 'A' circuit that is outside of the calibrated range. Typical range is 0.5V at idle to 4.5V at wide-open throttle (WOT). If the signal drops below 0.2V or exceeds 4.8V for a specific duration, the code is set. The ECM uses this data to calculate fuel delivery and ignition timing; a failure typically forces the vehicle into 'Limp Mode' to ensure safety by disabling electronic throttle control functionality.
Possible Causes
Faulty Throttle Position Sensor (TPS) or complete Throttle Body assembly
Wiring harness damage, chafing, or corrosion in the connector
Faulty Accelerator Pedal Position (APP) sensor
Internal Engine Control Module (ECM) failure
Symptoms
Recommended Solution
- STEP 1: Inspect the TPS/APP connector for corrosion or backed-out pins. Clean with contact cleaner.
- STEP 2: Verify reference voltage. With ignition ON, check for a steady 5.0V reference and a solid ground at the sensor connector.
- STEP 3: Perform a linear sweep test using a scan tool (live data) or oscilloscope. Ensure voltage increases smoothly from ~0.5V to ~4.5V without dropouts. Resistance should be checked against factory specs (typically 500-3000 Ohms).
- STEP 4: Clean the throttle body of carbon deposits. If the sensor is faulty and integrated, replace the throttle body or pedal assembly. Perform a throttle adaptation/idle relearn procedure after replacement.
Mechanic's Tip
Before replacing expensive components, always double-check wiring and connections. 80% of all sensor faults are actually due to corroded contacts or broken wires.
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