Can someone explain to me what the purpose of the Knock Sensor is? I've having to replace mine and just wondered what its job is.

Thanks

 

It detects knock i.e. detonation or pre-ignition in the cylinders, basically an uncontrolled burn. This is often heard by a tinkling noise from the outside just before something goes badly wrong. So by attaching a knock sensor to the engine block, the ECU can detect when it's happening and hopefully take steps to reduce it. It's similar to a microphone, but it only picks up sounds of one particular frequency range corresponding to a genuine knock signal in that particular engine. It then outputs a voltage according to how loud this signal is. There is a formula to calculate what the frequency of an engine's knock is likely to be depending on its cylinder size, but I can't remember what it is off the top of my head.