What causes a throttle position sensor high voltage code?

Most often the signal wire touching a supply voltage, or an open earth on the sensor circuit. Both push the signal to the top of its range without the sensor itself being faulty, which is why testing the earth before buying a throttle body is worth the five minutes. A failed sensor and water in the connector make up most of the remainder. On older cable throttles, a linkage genuinely stuck open will produce it honestly.

It should be treated as such. On an electronic throttle the car protects itself by restricting power, which leaves you unable to accelerate properly in traffic. On an older mechanical throttle, the possibility that the throttle really is held open matters much more, because a throttle that does not close when you lift off is an emergency. Check by hand that the throttle snaps shut against its stop before driving anywhere.

Will P0123 fail an MOT?

Yes. The engine management lamp it lights is a major defect for petrol cars first used from 1 July 2003 and diesels from 1 July 2008, with 371,859 recorded failures against it. A car held in limp mode may also be unable to hold the fast idle needed for the emissions reading, and an emissions test that cannot be completed is a further major failure.

Can a bad earth cause P0123?

Yes, and it is one of the more commonly missed causes. The sensor is a voltage divider between the reference supply and earth. If the earth path develops resistance, the signal voltage floats upward even though the sensor and the throttle are both fine. Measuring the voltage drop between the sensor's earth pin and the battery negative terminal will show it: anything much beyond a couple of tenths of a volt means the earth needs cleaning or repairing rather than the sensor replacing.

Powertrain code · standard

P0123Throttle position sensor signal too high

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

Last updated 27 July 2026

Quick answer

The throttle position signal has risen above the maximum the engine control unit accepts, suggesting the throttle is open further than it physically can be. The car will normally cut to a restricted throttle immediately.

How the car works this out

The sensor's output climbs as the throttle opens but is arranged to stop short of the full reference voltage, so there is always headroom at the top. P0123 is set when the signal exceeds that ceiling, typically above about 4.5 volts, or when it reads wide open while the pedal is released. On an electronic throttle the second position signal makes the fault obvious: the control unit has one sensor saying the throttle is shut and the other saying it is flat out, and it will always take the cautious interpretation.

What you might notice

Immediate reduced power or limp mode
Racing or hanging idle on cars with a mechanical throttle cable
Very rich running and poor economy
Harsh, early or unpredictable automatic gearchanges
The engine refusing to rev beyond a low limit

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 the five-volt reference or to battery voltageCommon
How to check: Unplug the sensor with the ignition on. If the reported throttle position stays at full rather than dropping, the short is in the loom rather than the sensor.
2An open or high-resistance earth on the sensor circuit, which pushes the signal to the top of its rangeCommon
How to check: Measure the voltage drop between the sensor earth pin and the battery negative. More than a couple of tenths of a volt with the ignition on indicates a poor earth.
3A failed sensor or throttle bodyPossible
4Water ingress into the throttle body or pedal connectorPossible
5On mechanical throttles, a sticking cable or linkage genuinely holding the throttle openLess common
How to check: Work the throttle by hand at the throttle body and feel for a linkage that does not snap shut against its stop. This is the one cause with a real safety dimension of its own.

What to do about it

1Check that the throttle is not physically stuck open before assuming an electrical fault, particularly on older cars with a cable. A throttle that does not return is dangerous in a way an electrical code is not.
2Unplug the sensor with the ignition on and see whether the reported position falls. That single test tells you whether to chase wiring or replace a sensor.
3Check the earth for the sensor circuit before condemning the throttle body.
4Inspect the connectors for water, which finds its way into throttle body plugs on cars parked outside through a wet winter.
5Relearn the throttle adaptation after any repair to the throttle body.

Can you keep driving?

Treat this one as a stop. On an electronic throttle the car will restrict itself and be very slow, which is a hazard in traffic. On an older mechanical throttle there is the additional possibility that the throttle really is stuck open, and a car that keeps accelerating when you lift off is an emergency, not a fault code. Have it looked at where it stands.

Will it fail an MOT?

The illuminated engine management lamp is a straight major defect, one of 371,859 recorded failures. Two further consequences on test day: a car restricted to a low rev limit may not reach the steady fast idle the emissions check requires, which is a separate major failure for an incomplete emissions test, and a throttle that is genuinely sticking open is a defect a tester will record on its own merits.

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

What causes a throttle position sensor high voltage code?: Most often the signal wire touching a supply voltage, or an open earth on the sensor circuit. Both push the signal to the top of its range without the sensor itself being faulty, which is why testing the earth before buying a throttle body is worth the five minutes. A failed sensor and water in the connector make up most of the remainder. On older cable throttles, a linkage genuinely stuck open will produce it honestly.
Is P0123 dangerous?: It should be treated as such. On an electronic throttle the car protects itself by restricting power, which leaves you unable to accelerate properly in traffic. On an older mechanical throttle, the possibility that the throttle really is held open matters much more, because a throttle that does not close when you lift off is an emergency. Check by hand that the throttle snaps shut against its stop before driving anywhere.
Will P0123 fail an MOT?: Yes. The engine management lamp it lights is a major defect for petrol cars first used from 1 July 2003 and diesels from 1 July 2008, with 371,859 recorded failures against it. A car held in limp mode may also be unable to hold the fast idle needed for the emissions reading, and an emissions test that cannot be completed is a further major failure.
Can a bad earth cause P0123?: Yes, and it is one of the more commonly missed causes. The sensor is a voltage divider between the reference supply and earth. If the earth path develops resistance, the signal voltage floats upward even though the sensor and the throttle are both fine. Measuring the voltage drop between the sensor's earth pin and the battery negative terminal will show it: anything much beyond a couple of tenths of a volt means the earth needs cleaning or repairing rather than the sensor replacing.

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