What Is P0120?
P0120 is a circuit-level fault on Throttle Position Sensor (TPS) circuit A. The TPS measures the angular position of the throttle butterfly and reports it to the ECU. The ECU uses TPS data for fuel injection, ignition timing, torque management, and traction control. P0120 means the TPS signal is outside the expected electrical range - not performing poorly within range, but absent or at an impossible voltage.
On modern drive-by-wire (DBW) throttle systems, two TPS sensors are fitted to the throttle body for redundancy - P0120 affects Circuit A. A TPS circuit fault typically causes a significant reduction in engine performance as the ECU enters a safety mode, limiting throttle opening to protect the drivetrain. The vehicle may enter limp-home mode with restricted power.
Common Symptoms
- Engine management light on
- Limp-home mode - restricted power
- Hesitation or poor throttle response
- Rough idle or stalling
- Possible traction control light illuminated
- Engine may be unresponsive to throttle input
Common Causes
How to Diagnose P0120
Check TPS Connector and Wiring
Inspect the TPS connector for corrosion, moisture, or damaged pins. Check supply voltage (typically 5V) and earth continuity at the connector.
Monitor TPS Signal in Live Data
With the engine running, read TPS percentage or voltage and move the throttle from closed to fully open. The signal should rise smoothly and proportionally from approximately 0.5V at idle to 4.5V at full throttle. Any dead spots, jumps, or out-of-range readings confirm a sensor fault.
Check for Throttle Body Carbon
Remove the air intake duct from the throttle body and inspect for heavy carbon deposits. Clean with throttle body cleaner if necessary - deposits can interfere with the TPS mechanism on some throttle body designs.
Test TPS Resistance
Measure resistance between the wiper and reference pins while slowly moving the throttle shaft. Resistance should change smoothly with no dead spots or open circuit points.
Replace TPS or Throttle Body Assembly
On many modern DBW throttle bodies, the TPS is integral to the throttle body and cannot be replaced separately. The entire throttle body assembly requires replacement. Cost: £80–£300 depending on vehicle.
Verdict
P0120 typically causes limp-home mode - a clear symptom. Check the connector first. If wiring and supply are good but the signal is absent or erratic, TPS or throttle body replacement is required. On DBW systems, always clear the throttle body adaptations after replacement and perform a relearn procedure.
Want the full picture? The OBD Fault Code Plain English Guide (PDF) covers the most common UK fault codes in one plain-English download.
