P0121: Throttle/Pedal Position Sensor/Switch A Circuit Range/Performance

What does P0121 mean?

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.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

• 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)

Common causes

• High Carboned or mechanically sticking throttle plate
• Medium Worn potentiometer track (signal sticks or jumps)
• Medium Air leak or faulty MAF/MAP sensor (model comparison)
• Medium Contact resistance in signal or ground wire
• Low Sensor or idle stop misadjusted (cable systems)

Likelihood is a general estimate across vehicles; your make and model may differ.