P0221
PowertrainThrottle Position Sensor/Switch B Circuit Range/Performance
The ECU watches the voltage coming back from the second throttle position sensor (circuit B) and compares it against engine speed and the other sensor signals. When that voltage drifts outside the window it expects for what the engine is actually doing, it logs P0221. Most modern petrol cars run two TPS signals as a safety cross-check, so when one disagrees with the other, the ECU doesn't trust either and often drops you into a reduced-power mode to keep things safe.
ⓘ Information only. This page provides general educational information about fault code P0221. 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 P0221 mean?
P0221 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Throttle Position Sensor/Switch B Circuit Range/Performance.
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 feeling fine, sometimes with it crawling
- • Hesitation or a flat spot when you press the accelerator
- • Limp mode: throttle response capped, car won't rev past a set point
- • Jerky or surging response off idle, the pedal feels disconnected from the engine
- • Stalling at idle or when coasting to a stop
- • Occasional puff of black smoke under acceleration if fuelling has gone wrong
Possible causes
Causes commonly associated with P0221, 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 B itself, common once the throttle body has racked up the miles and the internal track wears
- 2. Corroded, loose, or chafed wiring in the sensor circuit, the connector pins back at the throttle body are a classic spot for green corrosion
- 3. Water or muck inside the sensor connector, especially on cars parked outside or after a botched engine wash
- 4. Contaminated throttle body, carbon build-up that stops the plate sitting where the sensor expects
- 5. TPS knocked out of alignment after a recent throttle body or sensor swap
- 6. Pedal position sensor fault on drive-by-wire setups, since the ECU correlates pedal and throttle signals
- 7. ECU calibration or internal fault, rare, only worth considering once everything else checks out
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. Scan the car and note the freeze-frame data, engine speed, throttle angle and the conditions when it logged. That tells you whether it's a steady fault or an intermittent dropout.
- 2. Get on the wiring and connectors at the throttle body before you condemn any parts. Unplug it, look for corroded or pushed-back pins, water, and chafed insulation. Half of these are connector faults.
- 3. Put a multimeter on the sensor B signal: roughly 0.5V at closed throttle climbing smoothly to around 4.5V at wide open. Any sudden jumps or dead spots point to a worn sensor.
- 4. Better still, watch it on live data or a scope while you ease the pedal through its full travel slowly. A clean signal sweeps without glitches; a dropout will show as a spike to zero or 5V.
- 5. Check the 5V reference feed and the earth back to the ECU. A poor earth makes a perfectly good sensor read rubbish.
- 6. If the throttle body looks coked up, clean it and recheck. Carbon round the plate can throw the angle reading off enough to trip the code.
Common questions about P0221
If I clear the code, will it stay gone or just come back? +
Depends entirely on why it set. If it's a worn sensor track or a corroded connector, clearing it buys you a few miles at most before the light's back on. A genuine intermittent like a chafed wire might stay clear for days then return when the wire moves and breaks contact. Clearing it is a useful test, if it returns quickly the fault is still live, but it's never the fix on its own.
What am I risking if I just leave it and keep driving? +
The immediate risk is the car dropping into limp mode at a bad moment, on a slip road or pulling out of a junction, with the throttle suddenly refusing to open. Surging or stalling at idle is the other concern. It won't usually leave you stranded, but unpredictable throttle is a safety problem in traffic. Long term, if it's a fuelling-related cause you can foul plugs and waste fuel.
How quickly do I need to sort this? +
Sooner rather than later, mainly because of the limp mode and stalling behaviour rather than engine damage. If the car drives normally and only the light is on, you've got time to diagnose it properly over a week or two. If it's already cutting power or stalling, get it looked at before you rely on it for a motorway run or anything in heavy traffic.
Is it the sensor itself or just the wiring behind it? +
More of these turn out to be wiring and connector faults than dead sensors, which is why the connector inspection comes before buying parts. The pins at the throttle body corrode, the looms chafe, and water gets into the plug. That said, the sensor track does wear out on high-mileage throttle bodies. The multimeter and live-data tests tell you which: a clean sweep with a glitch only when you wiggle the loom is wiring, a sensor with dead spots in its travel is the sensor.
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 →