Why does a closed throttle not read zero volts?

By design. The sensor is set up so that even at the fully closed stop it still produces a small resting voltage, usually a few tenths of a volt. That deliberate offset is what lets the control unit tell the difference between a throttle that is genuinely shut and a signal wire that has broken or shorted to earth. When the voltage falls below that offset, the reading is impossible rather than merely low, and P0122 is stored.

Can I drive with P0122?

You should not. The control unit has lost a trustworthy throttle signal and responds by restricting the throttle, so the car may have almost no power and may stall repeatedly. That is not a comfort problem, it is a safety one: a car that will not accelerate when you pull out of a junction is a hazard. Have it recovered, or move it only over very short distances on quiet roads.

Will P0122 fail an MOT?

Yes, and probably on two counts. The illuminated engine management lamp is a major defect for petrol cars first used from 1 July 2003 and diesels from 1 July 2008, recorded 371,859 times. Beyond that, a car stuck in throttle limp mode usually cannot hold the fast idle the tester needs for the emissions reading, and an emissions test that cannot be completed is a major failure too.

Do I need to reset anything after replacing a throttle body?

Yes. Almost every electronic throttle needs its adaptation or basic setting relearned with a scan tool after being replaced or cleaned. The control unit stores a learned reference for the closed position and the idle airflow, and a new throttle body will not match the old learned values. Skipping the relearn typically leaves a hunting idle, a hanging idle when you lift off, or the code returning.

Powertrain code · standard

P0122Throttle position sensor signal too low

Standard definition: Throttle/Pedal Position Sensor "A" Low Input

Last updated 27 July 2026

Quick answer

The throttle position signal has dropped below the voltage the engine control unit accepts as possible, even with the pedal fully released. The car will almost always restrict power until the fault is fixed.

How the car works this out

A throttle position sensor is arranged so that its output never reaches zero, even at the closed-throttle stop — there is always a small resting voltage, typically a few tenths of a volt, precisely so the control unit can tell a genuinely closed throttle from a broken wire. P0122 is set when the signal falls below that resting value. The control unit reads that as an impossible position rather than a closed throttle, and because it can no longer trust what the throttle is doing, it defaults to a restricted opening.

What you might notice

Reduced power or limp mode, often from the moment the engine starts
Little or no response to the accelerator pedal
Rough idle or stalling
Engine that starts and immediately dies
Automatic gearbox stuck in a single gear or shifting harshly

What causes it

Most likely first. A code names the check that failed, not the part that failed it, so more than one of these can produce the same number.

1The signal wire shorted to earth or brokenCommon
How to check: Back-probe the signal wire with the ignition on and the pedal released. There should be a small but definite resting voltage; essentially zero points at the wiring rather than the sensor.
2The five-volt reference supply missing, often because another sensor sharing it has failed shortCommon
How to check: Check the reference voltage at the connector. If it is absent, unplug the other sensors on the same supply one at a time until it comes back.
3A corroded or water-damaged connector at the throttle body or the pedalPossible
4A failed sensor or a failed electronic throttle bodyPossible
5A chafed loom between the throttle body and the control unit, common where it crosses the engine and flexesPossible

What to do about it

1Have it recovered or driven very carefully. A car that will not respond to the pedal properly is difficult to place safely in traffic.
2Check the connector at the throttle body first for water, corrosion and pushed-back pins.
3Measure the reference voltage and the signal voltage at the connector. Those two readings separate a wiring fault from a dead sensor in a couple of minutes.
4Look for a shared-supply fault if other sensors are also reporting low input codes. Replacing the throttle body when a fuel pressure sensor is dragging the supply down is an expensive mistake.
5Run the throttle adaptation procedure after any repair to the throttle body, otherwise the idle will not settle.

Can you keep driving?

Not sensibly. With no trustworthy throttle signal the car is in restricted mode and may barely respond to the pedal, which makes pulling out of junctions and joining fast roads genuinely dangerous. Some cars stall repeatedly. Get it recovered or move it only the shortest distance on quiet roads.

Will it fail an MOT?

The lamp alone is a major defect and one of 371,859 recorded failures against an illuminated engine malfunction indicator lamp. The practical problem is bigger than the paperwork: a car locked in throttle limp mode often cannot be raised to the steady fast idle the emissions check requires, so the emissions test cannot be completed, which is a second major failure in its own right. There is no point presenting the car until this is repaired.

The light this turns on

Engine management lightThe engine management system has found a fault it cannot correct and has stored a code for it. The car will usually still drive, but an illuminated engine light is an automatic MOT failure on petrol cars first used from 1 July 2003 and diesels from 1 July 2008.

Common questions

Why does a closed throttle not read zero volts?: By design. The sensor is set up so that even at the fully closed stop it still produces a small resting voltage, usually a few tenths of a volt. That deliberate offset is what lets the control unit tell the difference between a throttle that is genuinely shut and a signal wire that has broken or shorted to earth. When the voltage falls below that offset, the reading is impossible rather than merely low, and P0122 is stored.
Can I drive with P0122?: You should not. The control unit has lost a trustworthy throttle signal and responds by restricting the throttle, so the car may have almost no power and may stall repeatedly. That is not a comfort problem, it is a safety one: a car that will not accelerate when you pull out of a junction is a hazard. Have it recovered, or move it only over very short distances on quiet roads.
Will P0122 fail an MOT?: Yes, and probably on two counts. The illuminated engine management lamp is a major defect for petrol cars first used from 1 July 2003 and diesels from 1 July 2008, recorded 371,859 times. Beyond that, a car stuck in throttle limp mode usually cannot hold the fast idle the tester needs for the emissions reading, and an emissions test that cannot be completed is a major failure too.
Do I need to reset anything after replacing a throttle body?: Yes. Almost every electronic throttle needs its adaptation or basic setting relearned with a scan tool after being replaced or cleaned. The control unit stores a learned reference for the closed position and the idle airflow, and a new throttle body will not match the old learned values. Skipping the relearn typically leaves a hunting idle, a hanging idle when you lift off, or the code returning.

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