P0122 Throttle Position Sensor A Circuit Low Input

Investigate soon

The voltage on this circuit is sitting near zero. Typically a short to earth, or a sensor that has failed to its low end.

Can you drive it? The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.

The definition

P0122: Throttle Position Sensor A Circuit Low Input.

P0122 is a powertrain code (the engine or gearbox), defined by the SAE standard - it means the same thing on any make of car, and any generic OBD-II reader will decode it.

What to check first

Cheapest and fastest checks first:

Unplug the sensor. If the reading jumps to full scale, the wiring is fine and the sensor has failed low.
Check the sensor's 5V supply and its earth before condemning the sensor itself.

What actually causes this on a VAG

On the electronic throttle bodies used across the VAG range, the throttle position sensor isn't a bolt-on part - it lives inside the throttle valve control module (often listed as J338), so a genuinely failed sensor means the whole throttle unit. "Low input" means the ECU is seeing less voltage on that signal than is physically possible, which points at a short to earth, a broken wire, or a missing supply to the sensor rather than anything mechanical. Expect the EPC light and reduced power while it's stored.

Most likely causes, in order

Connector or wiring at the throttle body. The plug sits in the hottest, most-disturbed corner of the engine bay, and either a damp, corroded connector or insulation rubbed through against something metal will drag the signal down. Unplug it and look for green corrosion, a pin pushed back in its housing, or shiny worn patches where the loom crosses a bracket or pipe.
Lost sensor supply from the ECU. The ECU feeds several sensors from the same regulated supply, so if this code arrived at the same moment as unrelated sensor codes - manifold pressure, pedal, camshaft - the supply or its earth is the fault, not the throttle. Compare the stored codes and their freeze-frame data before touching anything.
Failed throttle valve control module. Real, but it belongs last because it's the expensive answer. Only conclude this once you've proved the connector is clean, the wiring is intact and the sensor supply and earth at the plug are both healthy.

Worth knowing: Taken at face value this code sends people straight to a throttle body, because the sensor isn't sold separately. Prove the connector, the wiring and the sensor's supply and earth first - and whichever way it goes, the throttle must be adapted with a VAG-capable tool after any replacement or disturbance, or you'll simply have a fresh fault. Don't put off an intermittent one, either: the car can drop into limp mode with no warning, and losing power as you pull out or overtake is the genuine hazard with this code.

Reading the code is the easy part

A code names the circuit that complained. What tells you why is watching the sensor behind it while the engine runs — and that is where a code reader stops and a diagnostic tool starts.

VAGPULSE reads every module in a VW, Audi, Skoda or SEAT — not just the engine — shows the live measuring blocks behind this fault, gives you the freeze-frame conditions it was logged under, and clears it once you have fixed it. It also does what a generic OBD reader cannot on a VAG: service resets, coding, adaptations, and Stage 1–3 flashing with your original file saved first.

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Common questions

What does P0122 mean?

P0122 means Throttle Position Sensor A Circuit Low Input. The voltage on this circuit is sitting near zero. Typically a short to earth, or a sensor that has failed to its low end.

Can I drive with P0122?

The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.