what exactly is this knock sensor?where is it located?and what does it do?how do you know if you need to change it?i dont think mines has ever been changed

 

search button....shame on you, you should know better!

 

Its on the back of the block and under the intake manifold, if it has black crap leaking from it, its bad.

It pretty does what it sounds like, it picks up any noise from the engine, sends it to the pcm and the pcm backs off timing.