The engine’s knock sensor (or sensors) detects preignition and detonation, potentially damaging forms of abnormal combustion. Though full-blown knocking and pinging are audible to the human ear, the knock sensor detects imperceptible levels. When knocking is detected, the engine’s computer can retard the spark plug timing to prevent it. In the event of knock sensor failure, the driver may experience audible knocking or pinging and/or diminished acceleration, which is sometimes accompanied by decreased gas mileage. A check engine light is a likely result.