P0122
PowertrainThrottle Position Sensor/Switch A Circuit Low Input
Most of the time this is a cheaper fix than it sounds, either the throttle position sensor itself or the wiring feeding it. The ECU watches a voltage signal from the sensor (usually a 0 to 5V scale), and P0122 means that voltage has dropped below the bottom of the expected window. That low reading tells the ECU the throttle could be telling it nonsense, so it often drops the car into limp mode to protect things while you sort it out.
ⓘ Information only. This page provides general educational information about fault code P0122. 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 P0122 mean?
P0122 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Throttle Position Sensor/Switch A Circuit Low 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 with the car suddenly feeling gutless
- • Limp mode, with engine speed capped somewhere around 2,500 to 3,500 rpm
- • Throttle that feels dead or laggy, you press the pedal and nothing much happens for a beat
- • Lumpy or surging idle, sometimes sitting higher than it should
- • Stalling, usually as you slow down to a junction or roundabout
- • Worse fuel economy because the engine is running on a fallback map
Possible causes
Causes commonly associated with P0122, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Worn or failing throttle position sensor, the wiper track inside wears and the signal drops out. Common on older cable-throttle Fords and Vauxhalls
- 2. Damaged or corroded wiring and connectors at the sensor, road salt and engine bay heat take their toll over the years
- 3. A short to ground in the signal wire, which pulls the voltage straight to zero and trips the code instantly
- 4. Poor earth on the sensor circuit, giving a reading that drifts low under certain conditions
- 5. Faulty accelerator pedal position sensor on drive-by-wire cars, which can throw circuit A codes on the throttle side
- 6. ECU fault, rare and only worth considering once everything upstream checks out clean
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. Read the live data on a scanner and watch the TPS voltage as you slowly open the throttle by hand. It should sweep smoothly from roughly 0.5V to around 4.5V. A flat zero or a dropout mid-sweep tells you a lot
- 2. Check the connector and wiring at the sensor before anything else. Unplug it, look for green corrosion on the pins, wiggle the loom and watch for the reading to jump
- 3. Back-probe the reference feed with a multimeter, you want a steady 5V. No reference voltage means the problem is the supply, not the sensor
- 4. Check the earth at the sensor against a known-good ground. A bad earth gives a believable but wrong low signal
- 5. Swap in a known-good sensor if you have access to one, then clear the code and road test to see if it returns
- 6. If sensor, wiring, reference and earth all read correctly, the fault is pointing at the ECU or the pedal sensor on drive-by-wire setups
Common questions about P0122
Is it the sensor that's gone or is it more likely a wiring problem? +
It can be either, and the only way to know is to look at the voltage. If the signal drops out only when you move the throttle or wiggle the loom, that points to wiring or a tired connector, which is the cheaper fix. If the voltage is dead low and steady regardless of throttle position, and the 5V reference and earth both check out, then the sensor itself has usually failed internally. On higher-mileage cable-throttle cars the worn sensor track is the usual culprit. On newer drive-by-wire cars, check the connector and pedal sensor before you condemn anything.
How long does this take to put right? +
A straightforward TPS swap on a car where the sensor bolts to the side of the throttle body is often a 20 to 30 minute job once you have the part. The bulk of the time on these is diagnosis, finding a chafed wire or a corroded pin in the loom can eat an hour or more of probing. If the whole electronic throttle body needs replacing, expect a couple of hours including the relearn procedure some cars need afterwards.
Should I save money with an aftermarket sensor or pay for genuine? +
A quality branded aftermarket sensor from a name like Bosch, Delphi or Hella is fine for most everyday cars and costs a good bit less than main dealer price. Where I'd be careful is the no-name eBay specials, the resistive track on the cheapest ones wears out fast or reads inconsistently, and you end up doing the job twice. On drive-by-wire throttle bodies the ECU can be fussier about the part it sees, so on a newer or premium car I'd lean towards OEM or a top-tier aftermarket brand rather than the cheapest thing in the listing.
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 →