What is a Crankshaft Position Sensor?
Tells the ECU exactly where the crankshaft is — without it, the engine won't start.
Simplified animation — not to scale.
In plain English
A magnetic or Hall-effect sensor mounted near the crankshaft that detects the passage of teeth on a reluctor ring, giving the ECU precise crank position and engine speed.
"Like a referee with a stopwatch counting laps on the crankshaft — without it, the engine has no idea when to fire."
How it works
A toothed wheel (reluctor) bolted to the crankshaft passes the sensor as the engine spins. Each tooth disturbs the magnetic field, producing a voltage pulse. One or two teeth are deliberately missing to mark top-dead-centre. The ECU counts pulses to measure RPM and uses the gap to know exactly where cylinder 1 is, so it can fire injectors and ignition coils at precisely the right moment. No signal means no spark and no fuel — the engine simply won't run.
Signs of trouble
- ⚠Engine cranks but won't start
- ⚠Sudden stalling at any RPM
- ⚠Check engine light (P0335)
- ⚠Intermittent misfires or tachometer reading zero
£65–£230
Always get a written quote. Prices vary by car, region, and parts brand.