HOTSYSTEM New Knock Sensor for Subaru Knock Sensor Forester Legacy Impreza Outback 2.2L 2.5L The drastic shocking caused by the following reasons: ignite too early ,the exhaust circulation is bad,the Low standard fuel will lead to damaged of the engine. The knock sensor provides knock signal to the computer, which makes the computer re-adjust the ignition timing to prevent further knock. In fact,they play a role in oxygen sensor to feedback control loop ignition timing