P0120 — Throttle Position Sensor Circuit A Fault

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

Failed TPS SensorThe sensor potentiometer element has worn or failed, producing a signal outside the valid voltage range. Common on high-mileage vehicles with frequent throttle movement.

Damaged Wiring HarnessThe TPS wiring loom runs near hot or moving components. Chafing or heat damage causes breaks in the signal, earth, or supply wires.

Corroded ConnectorMoisture ingress into the TPS connector causes pin corrosion, creating high resistance or open circuits in the sensor circuit.

Throttle Body ContaminationHeavy carbon deposits inside the throttle body can physically impede the throttle plate and the TPS shaft, causing mechanical binding that results in electrical signal anomalies.

ECU Input FaultRare — the ECU TPS input circuit has failed. Diagnose only after all external circuit checks have been completed.

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.

Frequently Asked Questions

Drive-by-wire (DBW) replaces the mechanical throttle cable with an electronic accelerator pedal sensor and an electric throttle motor. The ECU reads the pedal position and commands the throttle motor. The TPS is the feedback sensor confirming the throttle has reached the commanded position.

The ECU uses TPS data for torque management and emissions control. Without a valid throttle position signal, it cannot safely control engine output, so it restricts power to a safe minimum to protect the drivetrain and allow the driver to continue at low speed.

For short distances only. Limp-home mode typically limits engine speed to 2,500–3,000rpm and power to around 30–50% of normal. It is designed to get you to a garage, not for normal use.

On most modern vehicles, yes. After replacing the throttle body or TPS, a closed throttle adaptation procedure is required using a scan tool. Without it, idle quality and throttle response may be poor.

It depends on which warning light the code is triggering. Since 2018, any car presenting with an illuminated amber Engine Management Light (EML) at the MOT is a Major failure under DVSA rules — even if the car drives perfectly. A red warning light is always a Major or Dangerous failure depending on context. If clearing the fault makes the light go out and the code does not reappear during the pre-test drive, you will pass; if the code returns within minutes of clearing, the underlying fault must be fixed before MOT day. A tester is required to fail the car on the light being on, regardless of whether the underlying fault is something safety-critical or not. For codes that affect emissions specifically (catalyst, lambda, EGR), the car may also fail the actual emissions check. Fix the cause, clear the code, and drive the car for a few miles before the test.

P0120 — Throttle/Pedal Position Sensor A Circuit