The operation of the Knock Sensor (KS) is to monitor preignition or "engine knocks" and send the signal to the PCM. The PCM responds by adjusting ignition timing until the "knocks" stop. The sensor works by generating a signal produced by the frequency of the knock as recorded by the piezoelectric ceramic disc inside the KS. The disc absorbs the shock waves from the knocks and exerts a pressure on the metal diaphragm inside the KS. This compresses the crystals inside the disc and the disc generates a voltage signal proportional to the frequency of the knocks ranging from zero to 1 volt.
There is real no test for this sensor, the sensor produces it's own signal based on information gathered while the engine is running. The sensors also are usually inaccessible without major component removal. The sensors can be monitored with an appropriate scan tool using a data display or other data stream information. Follow the instructions included with the scan tool for information on accessing the data. The only test available is to test the continuity of the harness from the PCM to the sensor.