I found a few things on the net about them, and was wondering if anyone found them to be interesting....
the first:
knock sensors on engines are set to detect knock at a certain frequency range, and shaving as little as 0.5 mm from the cylinder head can render the knock sensor completely ineffective as the change in combustion chamber also changed the frequency of the detonation, and they know this because when they stripped the motor and saw the detonation damage, they knew they had not recorded detonation via the knock sensors and I am sure this would not be widely known
The second:
A knock sensor can usually be tested by rapping a wrench on the manifold near the sensor (never hit the sensor itself!) and watching for the timing change while the engine is idling. If the timing fails to retard, the sensor may be defective -- or the problem may be within the electronic spark timing control circuitry of the computer itself. To determine the cause, you'll have to refer to the appropriate diagnostic chart in a service manual and follow the step-by-step test procedures to isolate the cause. Sometimes a knock sensor will react to sounds other than those produced by detonation. A noisy mechanical fuel pump, a bad water pump or alternator bearing, or a loose rod bearing can all produce vibrations that can trick a knock sensor into retarding timing.
Any thoughts??