How do knock sensors work and how does the ECU respond to knock?

Knock sensors are piezoelectric accelerometers that detect the high-frequency vibrations (typically 6-15 kHz) characteristic of engine knock. The ECU monitors sensor output during specific crankshaft angle windows when knock would occur, using bandpass filtering and signal processing to distinguish knock from normal engine noise. When knock is detected, the ECU retards ignition timing for the affected cylinder(s) until knock stops, then gradually advances timing to maintain optimal performance. Multi-cylinder individual knock control optimizes each cylinder independently.