GooDeal Knock Sensor for Subaru Forester Legacy Impreza Outback 2.2L 2.5L The knock sensor is a listening device located inside the engine near the intake manifold or the pistons that is designed to detect pressure inside the engine. The vibrations that it records as a result of this pressure are converted into voltage and sent to the car's engine control unit (ECU). This is to prevent detonation in the engine when the fuel/air mixture explodes too quickly and causes engine knock or "pinging." The ECU takes the information provided by the knock sensor and adjusts the ignition timing to the engine. Shipping from our USA warehouse, fast shipping and good service.