Knock sensors are used to detect knock, a sign of a combustion issue. Some engines have just one, others have two (commonly V engine layouts), and more sensors are occasionally used, but not common.
Most knock sensors are physically just a housing for a piezoelectric crystal, which generates some electrical current when squeezed. Typically, the knock sensor is screwed into the engine block or cylinder head, close to where combustion happens. When a knock occurs, the sound wave travels through the metal block and through the knock sensor housing. As the sound wave passes through, the housing deforms slightly, squeezing the crystal, generating a small electrical signal that the ECU can understand.
Importance of Torque When Installing Knock Sensors
Because of the way the piezoelectric crystal inside the sensor works, tightening the sensor to the proper torque has a direct impact on the sensor's performance. If it isn't tight enough, it might not pick up vibrations properly. If the sensor is installed too tightly, the crystal will constantly be squeezed, and vibrations passing through it will have little to no effect.
How much torque is required for a given knock sensor will differ from engine model to engine model. You will need to find the right torque for your application by looking it up or getting the information from the manufacturer.
P0325 Knock Sensor Diagnosis
In this video, Eric of EricTheCarGuy shows the diagnostic process when dealing with a P0325 code. This video was released on April 30th, 2012.
P0325 Knock Sensor Diagnosis Follow-Up
In this follow-up video, Eric of EricTheCarGuy addresses some of the comments he got on the other video. This video was posted about a week after the first one, on May 7th, 2012.