P0121
Throttle/Pedal Position Sensor "A" Range/Performance
The throttle position sensor (TPS) signal does not match the expected range for the commanded pedal position. On drive-by-wire vehicles, two redundant sensors are compared; a divergence between them or implausible motion triggers this code.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: medium
Symptoms
- Check engine light on
- Reduced throttle response, possible limp mode
- Hesitation or surging
- Idle too high or too low
Common causes
Ordered by how often each turns out to be the cause.
- TPS contact track worn (older potentiometer-type sensors)Common
- Carbon buildup blocking throttle platePossible
- Corroded TPS connectorLess likely
Parts involved
- Throttle Position Sensor
- Throttle Body
- Accelerator Pedal Sensor
P0121 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.