P0121Throttle/Pedal Position Sensor/Switch A Circuit Range/Performance
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- Needs a drive cycle to confirm a fix
- How the car detects it
- Sensor A is, in practice, throttle position sensor 1 at the throttle body; the accelerator pedal has its own codes (P2120 onwards). With a cable-operated throttle there is a single potentiometer (about 0.5 V closed to 4.5 V wide open). Electronic throttles have two sensors sharing a 5 V reference and ground, usually running in opposite directions – on GM throttle sensor 1 falls from 3–4 V at idle to below 1 V at full throttle. P0121 is a plausibility fault: the signal is electrically valid but does not fit the engine's state. Manufacturers check this very differently. GM compares throttle position against air mass and pressures modelled from the other intake sensors and needs two trips; Saab/GM 2011 cut power. Ford used P0121 only on its older second-generation electronic throttle, for more than 7° disagreement between the two sensors, without MIL – newer Fords report that as P2135. Cable throttles (Saab 1997) check that idle voltage stays between 0.24 and 1 V. Carbon on the throttle and worn sensor tracks are typical causes.
- What you may notice
- Check engine light on
- Limp mode with reduced power ('Reduced Engine Power') on electronic throttles
- Rough or too high idle
- Hesitation, delayed throttle response
- Shift problems on automatics (wrong load signal)
- Likely causes
- LikelyCarboned or mechanically sticking throttle plate
- PossibleWorn potentiometer track (signal sticks or jumps)
- PossibleAir leak or faulty MAF/MAP sensor (model comparison)
- PossibleContact resistance in signal or ground wire
- RareSensor or idle stop misadjusted (cable systems)
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Often possible to do yourself.Typical cost: €20 – €500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.