The engine knock sensor in your Subaru Forester is designed to detect high-frequency vibrations caused by irregular detonation (combustion). Abnormal combustion, or misfiring of the engine, causes the sensor to produce a "chattering" sound. A faulty knock sensor will cause the check light to come on, signaling a combustion problem. If the knock sensor is malfunctioning, this can lead to both poor fuel efficiency and major engine damage if not replaced immediately. Replacing the knock sensor on a Subaru Forester is a somewhat simple repair to perform.
Raise the hood of the vehicle and locate the knock sensor. Make sure the engine is cold. The sensor is mounted on the upper rear side of the engine block, just below the intake manifold pipe.
Remove the vacuum tube electrical connection by hand. Take out the single mounting bolt located in the center of the sensor using a socket wrench. Gently tap the sensor to break the seal.
Take off the knock sensor. Scrape away the built-up debris on the engine block using a razor or putty knife.
Mount the new knock sensor on the engine block. Tighten the center bolt using the socket wrench. Reconnect the vacuum tube electrical connection.
Start the engine and allow it to run for several minutes. Test-drive the vehicle, making rapid accelerations. Listen for signaling from the sensor. Check to see if the "check engine" light remains off.
