Throttle position sensor: P0121, P0122, P0123 and P2135

The car jerks pulling away, the revs live their own life, the pedal answers late. One reading on screen tells whether the throttle is to blame.

The control unit must know exactly how far the throttle plate is open. If the reading lies or jitters, it injects the wrong amount of fuel — hence jerks pulling away, flat spots during acceleration and an unstable idle.

What to look at first

Open throttle position and do not touch the pedal. With the pedal released the value must sit on one number. If it wanders by even a couple of per cent, the unit recomputes fuelling each time — and the revs swing after it.

Then press the pedal smoothly to the floor and release. The value should rise and fall evenly, without steps or dropouts. A dropout mid-travel is a worn track inside the sensor, the most common mechanical cause.

What the codes mean

P0121 — the reading does not match the rest of the picture: the unit compares the throttle with air flow and revs and sees a mismatch.

P0122 and P0123 — the signal has gone to the floor or the ceiling. That is no longer «lying a little» but an open or a short; check the connector and the wiring.

P2135 — modern cars have two position sensors, and they must confirm each other. A disagreement is treated as dangerous, and the car often drops into limp mode.

Dirt or failure

On an electronic throttle a common cause is soot around the edge: the plate does not reach its own zero, and the unit thinks the throttle is slightly open. Cleaning helps, with a caveat: afterwards the unit must relearn the closed position. Without that the idle can get worse than before — and that is expected behaviour, not a second fault.

Step by step in the guide unstable idle.