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
1998-2003 for BMW 540i 4.4L
2004-2005 for BMW 545i 4.4L
2006-2010 for BMW 550i 4.8L
2004-2005 for BMW 645Ci 4.4L
2006-2010 for BMW 650i 4.8L
1997-2001 for BMW 740i 4.4L
1997-2001 for BMW 740iL 4.4L
2002-2005 for BMW 745i 4.4L
2002-2005 for BMW 745Li 4.4L
2006-2008 for BMW 750i 4.8L
2006-2008 for BMW 750Li 4.8L
2007-2008 for BMW Alpina B7 4.4L
2000-2003 for BMW M5 4.9L
2000-2003 for BMW M5 5.0L
2000-2006 for BMW X5 4.4L
2002-2003 for BMW X5 4.6L
2004-2009 for BMW X5 4.8L
2003 for BMW Z8 4.8L
2001-2003 for BMW Z8 4.9L
2000-2003 for BMW Z8 5.0L