What is a Knock Sensor?
A tiny microphone on your engine block that listens for harmful knocking sounds and tells the ECU to back off the timing.
Simplified animation — not to scale.
In plain English
A piezoelectric sensor screwed into the engine block that detects the high-frequency vibrations caused by abnormal combustion — known as knocking or detonation. When it hears trouble, it sends a voltage signal to the ECU so the engine can adjust.
"Like a smoke detector for your engine — it listens for the tell-tale ping of bad combustion and alerts the computer before damage happens."
How it works
Under normal combustion, the flame front spreads smoothly through the cylinder. But if the air-fuel mixture ignites unevenly — often from heat, poor fuel, or too much boost — it creates pressure shockwaves that hammer the piston and cylinder walls. These shockwaves vibrate through the metal engine block at a specific high frequency. The knock sensor, mounted on the block, contains a piezoelectric crystal that generates a tiny electrical voltage when squeezed by these vibrations. The ECU constantly monitors this signal. When it exceeds a threshold, the ECU retards ignition timing (delays the spark) to reduce cylinder pressure and eliminate the knock. Once the knocking stops, timing gradually advances again.
Signs of trouble
- ⚠Check engine light on with P0325 or similar codes
- ⚠Engine hesitating or lacking power under load
- ⚠Pinging or rattling sound especially when accelerating
- ⚠Poor fuel economy
- ⚠Engine running hot
£65–£180
Always get a written quote. Prices vary by car, region, and parts brand.