What are knock sensors and where are they located?

 

They are located on the lifter valley cover, the plate directly below the intake manifold. They sense vibrations in the block and decide whether or not they think it is knock and tell the computer what it comes up with. The PCM adjusts timing accordingly.

 

They "sense" denotation inside the cylinders.
Denotation is when 2 flame fronts collide
The collision creates a knock.
This knock can be harmful to the tops of the pistons and valves.
The computer reads the knock and retards the timing
trying to reduce the knock.

Caused by couple of things.....
Advanced timing,
low grade fuel ( which burns at a quicker rate than higher octane fuel - higher octane fuel actually burns slower, which creates more power. )
Carbon build up on the tops of the pistons or
a sharp point on the top of the piston or head - creates a hot spot - the hot spot creates ingition under compression plus the plug fires - thus knock...

 

They're the overly sensitive devices that tell the PCM to turn "ON" the SES MIL after you install an aftermarket bumpstick. Pull off your intake and you'll see these two round black things with a wire coming out of them. They're a necessary part of an electronically controlled engine. They will prompt the PCM to reduce timing in case of knock.

 

Haven't seen that happen.