What is a Throttle Position Sensor?
A potentiometer that tells the ECU exactly how far you're pressing the accelerator.
Simplified animation — not to scale.
In plain English
A small sensor attached to the throttle body shaft (or to the pedal assembly on drive-by-wire systems) that converts butterfly angle into a 0–5V signal.
"Like the volume dial on a stereo — turn it slowly and the engine matches you; if the dial is broken the music skips or cuts out."
How it works
Inside is a resistive track and a wiper that moves with the throttle shaft. At closed throttle the output sits around 0.5V; at wide-open throttle it climbs smoothly to about 4.5V. The ECU combines this with MAF and RPM data to decide how much fuel to inject and when to fire the spark. Modern electronic throttles have two redundant sensors and the ECU compares them constantly — any mismatch triggers limp mode to keep the car safe.
Signs of trouble
- ⚠Hesitation or surging under acceleration
- ⚠Sudden idle drop or stall
- ⚠Limp mode with reduced power
- ⚠Check engine light (P0120 / P0121)
£55–£210
Always get a written quote. Prices vary by car, region, and parts brand.