Throttle/Pedal Position Sensor A Circuit
The Powertrain Control Module (PCM) detected a malfunction in the circuit for Throttle/Pedal Position Sensor or Switch 'A'. This code is set when the voltage signal from the sensor falls outside the expected normal operating range or fails to correlate properly with engine operating conditions.
This code is considered high severity because a failing throttle position sensor can cause unpredictable engine behavior, stalling, or force the vehicle into a restricted 'limp mode'. It should be diagnosed and repaired immediately to ensure safe driving.
Common symptoms
- Check Engine Light illumination
- Engine hesitates or stumbles during acceleration
- Vehicle enters 'limp mode' with restricted power and speed
- Rough or erratic engine idle
- Engine stalling when coming to a stop or pressing the throttle
Likely causes
- Defective Throttle Position Sensor (TPS) or Pedal Position Sensor
- Damaged, corroded, or loose wiring and connectors in the TPS circuit
- Carbon buildup or physical binding in the throttle body
- Incorrectly adjusted Throttle Position Sensor (primarily on older models)
- Faulty Powertrain Control Module (PCM)
What to check before replacing parts
Perform a visual inspection of the TPS wiring harness and connector
Look for: Look for frayed wires, corrosion, bent pins, or a loose connection that could cause an open or short circuit.
Monitor the TPS signal voltage using a scan tool while slowly pressing the accelerator
Look for: Look for a smooth, linear voltage sweep from approximately 0.5V at idle to 4.5V at wide-open throttle without any dropouts, spikes, or dead spots.
Test the TPS 5-volt reference and ground circuits with a multimeter
Look for: With the key on and engine off, verify there is a steady 5 volts on the reference wire and near 0 ohms of resistance on the ground wire.
Test the continuity of the TPS signal wire between the sensor connector and the PCM
Look for: Look for less than 1 ohm of resistance on the signal wire and ensure there is no continuity to ground or power, indicating the wire is intact and not shorted.
Inspect the throttle body for physical binding or excessive carbon buildup
Look for: Look for heavy carbon deposits or mechanical damage preventing the throttle plate from moving smoothly, which can physically restrict movement and skew sensor readings.
This page explains the generic P0120 definition and is general reference information, not a diagnosis for your specific vehicle. Actual causes, severity, and repairs vary by make, model, engine, and mileage — always confirm with a scan tool and a qualified technician before replacing parts.
Gear guide: Best OBD2 ScannersRead this code yourself next time — from $20 Bluetooth dongles to bidirectional scan tools.Rather have a pro handle it? Find a vetted local mechanic.