A knock sensor is like a small equipment microphone that is usually attached to the engine block and listens for a couple of things that a engine might do. One thing it listens for is pre-ignition or knocking inside the engine usually caused by a few issues including dirty gas, wrong engine timing low octane fuel, wrong air / fuel mixture, and also by the wrong spark plugs. It also listens for engine run on which happens when the vehicle is turned off, but tries to keep running until it stalls. Once a noise is detected, the knock sensor sends a signal to the cars computer which in turn will retard the timing at 2 degree intervals until it corrects the issue. If the knock sensor fails, it can seriously damage the engine, as continuous pre-ignition could cause melting or fractures in the pistons just to name a few.

Fit for
1999-2002 for Subaru Forester 2.5L
1999-2001 for Subaru Impreza 2.2L
1999-2002 for Subaru Impreza 2.5L
1999-2001 for Subaru Legacy 2.5L
2000-2001 for Subaru Outback 2.5L