Knock — e knock / detonation

Pre-ignition combustion event where the air-fuel mixture ignites before the spark plug fires. Detected by a piezo sensor on the engine block. Caused by too-much-timing, too-lean mixture, too-much-boost, low-octane fuel, or high engine temperature.

Knock is engine-protective: the ECU pulls timing to suppress it. Aggressive tunes that push close to the knock threshold are detected by the knock sensor and pulled back — but if the sensor fails, the engine eats itself. NEVER suppress knock DTCs.