P0123
PowertrainThrottle Position Sensor/Switch A Circuit High Input
You'll usually notice the car doing something odd with the throttle before you ever see the light. The accelerator might feel dead one second then surge the next, the idle hunts up and down, and on a bad day the car drops into limp mode and refuses to pull. Behind all that, the ECU is reading a voltage from the throttle position sensor that's far too high, up near the 5V supply line, when it should be sitting around 0.45V at closed throttle. The ECU can't trust that number, so it flags P0123 and often pulls power back to keep things safe.
ⓘ Information only. This page provides general educational information about fault code P0123. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0123 mean?
P0123 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Throttle Position Sensor/Switch A Circuit High Input.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine warning light on, often alongside the throttle acting strangely
- • Hesitation or a jerky kick when you press the pedal, like the throttle can't make its mind up
- • Idle that won't settle, sometimes hanging higher than normal
- • Car drops into limp mode with acceleration capped, common once the ECU stops trusting the sensor
- • Stalling, usually when coming to a stop or lifting off the throttle
- • Heavier fuel use because the engine is running off a default value instead of real throttle data
Possible causes
Causes commonly associated with P0123, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Worn or failed throttle position sensor. The internal track wears with mileage and the wiper can lose contact, sending the signal to full voltage. Common on higher-mileage cars
- 2. Corroded or moisture-damaged connector at the sensor. Water gets into the plug, the pins corrode, and the signal reading climbs
- 3. Damaged or chafed signal wire shorting to a 5V or battery supply, which pins the reading high
- 4. Poor ground in the sensor circuit, letting the signal voltage float upward with nothing holding it down
- 5. Carbon build-up around the throttle plate causing the plate and sensor to read out of range, more of an issue on cars without regular throttle body cleaning
- 6. Faulty ECM misreading the signal. Rare, and only worth considering once the wiring and sensor are proven good
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Back-probe the sensor signal wire with a multimeter and watch the voltage as you open the throttle by hand. It should climb smoothly from roughly 0.45V closed toward 5V at full open. A reading stuck near 5V or jumping straight to it points at the sensor or a short
- 2. Unplug the connector and check it properly for green corrosion, bent pins, and water. This is where a lot of P0123 faults actually live, so don't rush past it
- 3. Check for related codes first. A P0223 or P2135 stored alongside changes how you read the data, and an ECM that's already in limp mode behaves differently on test
- 4. Inspect the signal wire run from the sensor back toward the ECM for chafing, melted insulation, or any point it could be touching a 5V feed
- 5. Test the sensor ground for continuity and a clean connection. A poor earth here mimics a failed sensor exactly
- 6. Clear the code, road-test, and watch live data on a scanner to confirm whether the high reading comes back under real driving
Common questions about P0123
Can I keep driving the car like this, or should I park it up? +
I'd not run errands in it. The danger with P0123 isn't the light, it's the unpredictable throttle. The car can drop power without warning or surge when you don't want it to, and that's no good pulling out of a junction or merging on the motorway. If it's in limp mode it'll be sluggish but stable enough to limp home or to a garage at low speed. Don't ignore it for weeks though, a stuck-high signal can leave you stranded when it gives up completely.
Will this stop the car passing its MOT? +
The code on its own isn't a direct fail item, but the throttle position sensor is part of the emissions and engine management system. What the tester sees is the engine warning light. If the MIL is lit when you present the car, that's an MOT fail under the warning light check, full stop. Fix the cause, drive a few cycles to make sure the light stays off, then book the test.
What's it likely to cost to sort out? +
If it turns out to be the sensor itself, you're usually looking at a low three-figure bill at an independent garage, with the part often £30 to £120 and an hour or so of labour on top. Connector or wiring repairs are cheaper on parts but eat diagnostic time, so the bill can creep up depending on how long the fault hides. A full throttle body replacement on cars with an integrated sensor pushes you toward mid three figures. A main dealer will charge more per hour and bill more diagnostic time, so expect to pay noticeably above an independent for the same job.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →