HOTSYSTEM New Knock Sensor for 1997-1999 Subaru Knock Sensor Legacy Impreza 2... 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