Location

The knock sensor resides at the very top of the block, directly under the intake manifold on B series engines.

Here the intake manifold is not fitted although you do not have to remove the inlet to replace the sensor. B series engines without knock sensor as standard do not have the fitting hole tapped.

 

Purpose

To quite literally "listen" to vibrations going through the engine block. It is basically a peizo electric microphone, tuned to a specific frequency range.