Knock Sensor

Knock (detonation) happens when part of the mixture ignites by itself after the spark, causing a pressure spike that sounds like pinging. Prolonged heavy knock can damage pistons. A knock sensor bolted to the engine block listens for the specific vibration frequencies of knock. The control unit then retards the ignition on the affected cylinder until the knock stops.

The sensor is a piezoelectric device bolted to the engine block. Larger engines may have two or more sensors. Knock, also called pinging or detonation, happens when part of the mixture ignites on its own after the spark, creating pressure waves that hit the piston and cylinder walls. Light knock is usually controlled by the engine; heavy, repeated knock can damage pistons, rings and bearings.

A piezoelectric element in the sensor generates a small voltage when it vibrates. The control unit filters the signal and only looks for knock in a window just after each cylinder fires. The control unit filters the signal for the frequency typical of knock and retards timing on the affected cylinder until the knock stops.

Codes like P0325 point to the sensor or wiring. With a faulty sensor, many engines retard the timing as a precaution, which costs power and economy. A faulty knock sensor can make the control unit use conservative timing, reducing power and economy. The sensor must be tightened to the specified torque to work properly.