P0120
PowertrainThrottle Position Sensor/Switch (TPS) A Circuit Malfunction
You press the accelerator and the car hesitates, surges, or just drops into a slow safe mode where it won't rev past a couple of thousand RPM. That's the usual day-to-day picture with P0120. Behind it, the ECU is seeing a voltage from the throttle position sensor that doesn't make sense, either too high, too low, or jumping around. On most modern cars this sensor lives inside the electronic throttle body and tells the ECU exactly how far the throttle plate is open, so when its signal goes wonky the engine management throws up its hands and limits power to keep things safe.
ⓘ Information only. This page provides general educational information about fault code P0120. 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 P0120 mean?
P0120 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Throttle Position Sensor/Switch (TPS) A Circuit Malfunction.
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, sometimes with the car dropping into limp mode straight away
- • Hesitation or a flat spot when you put your foot down, then a sudden surge as the signal recovers
- • Rough or hunting idle, especially when cold or sitting at a junction
- • Stalling when you lift off the throttle or come to a stop
- • Sluggish acceleration with the revs capped, often around 2,000-2,500 RPM in limp mode
- • Worse fuel economy because the mixture is being calculated off a duff throttle reading
Possible causes
Causes commonly associated with P0120, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Worn throttle position sensor, the most common cause. The internal track wears a dead spot at the position your throttle sits most often, so the voltage glitches there
- 2. Corroded or loose connector at the throttle body, very common on older VAG cars and anything that's had water sitting in the engine bay
- 3. Damaged wiring in the TPS circuit, often where the loom flexes near the throttle body or rubs on a bracket
- 4. Heavy carbon build-up in the throttle body fouling the throttle plate so it doesn't sit or move where the ECU expects
- 5. Failed 5-volt reference or ground feed to the sensor, which drags the signal out of range
- 6. Faulty accelerator pedal sensor on drive-by-wire setups confusing the ECU's throttle logic
- 7. Less often, a failing throttle body motor or a software calibration that needs a relearn after work
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. Plug in a scanner and watch the live TPS voltage while you slowly open the throttle by hand with the key on, engine off. A healthy sensor sweeps smoothly from roughly 0.5V closed to about 4.5V wide open. Any flat spots, dropouts, or spikes point straight at the sensor
- 2. Wiggle the connector and harness near the throttle body while watching that live data. If the voltage jumps when you move it, you've found a wiring or connector fault, not a dead sensor
- 3. Back-probe the connector with a multimeter and confirm you've got a steady 5V reference and a clean ground. No reference voltage means the problem is upstream of the sensor
- 4. Check the throttle body for carbon. If the plate is gummed up, clean it with proper throttle body cleaner and recheck before condemning anything
- 5. Pull any other stored codes. P0121 to P0123 and pedal codes like P2135 often sit alongside P0120 and tell you whether it's a range or a correlation issue
- 6. If sensor sweep, supply, and wiring all check out, the throttle body or pedal sensor itself is the likely culprit, and many need a throttle relearn after fitting
Common questions about P0120
How do I know if it's the sensor itself or just dodgy wiring and a bad connector? +
The wiggle test settles it. With the scanner showing live TPS voltage and the key on, open the throttle gently and watch the signal sweep. If it climbs smoothly with no drops, the sensor is probably fine and you should be hunting a wiring fault. Now grab the connector and the loom near the throttle body and waggle it. A signal that flickers or cuts out when you touch the harness is a connector or wiring problem, often just corrosion in the plug or a chafed wire. A sensor with a worn track gives a clean dropout at one specific throttle angle every single time, usually around the position you drive at most. Worth a good look before buying parts, because a £5 clean-up of a corroded pin saves you fitting a sensor that was never broken.
How long does this actually take to put right? +
If it's a TPS or full throttle body swap, you're looking at well under an hour of labour on most cars, since it's usually a few bolts and a connector. The catch is the relearn afterwards. A lot of electronic throttle bodies need an idle relearn or adaptation reset before the car runs properly, and that's where it can eat time if your scanner can't do it and you end up at a garage. Tracing an intermittent wiring fault is the slow job and can swallow a couple of hours of poking around the loom. A throttle body clean is a quick afternoon task and worth trying first.
Is a cheap aftermarket throttle body or sensor worth fitting, or do I stick with genuine? +
For the standalone TPS, a decent quality aftermarket sensor from a known brand is fine and a fraction of the dealer price. Skip the no-name eBay specials though, they're a common cause of the code coming straight back because the voltage track is rubbish from new. For complete electronic throttle bodies it's a closer call. The cheap full assemblies can work, but some ECUs are fussy about non-genuine units and won't adapt to them cleanly, leaving you chasing your tail. On a higher-mileage everyday car a quality aftermarket throttle body is sensible money. If it's a fault-prone setup like some VAG petrol engines, a genuine or OE-equivalent unit tends to behave itself better long term.
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 →