Knock Sensors (KS) detect engine pre-detonation. They have 2 wires, one of which supplies a 5-volt reference and the other one supplies a return signal from the KS Sensor to the Powertrain Control Module (PCM). The sensor signal wire sends the data to the PCM when a knock occurs, as well as the degree. Then, the PCM delays the spark timing, thus preventing pre-detonation.
When the DTC P0333 OBDII code is set, it indicates that there is a high voltage at the output of the KS Sensor. This is greater than 4.5V, although this value may vary depending on vehicle make and model.