How to Replace the Knock Sensor for the Subaru Forester

Written by brett johnson
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How to Replace the Knock Sensor for the Subaru Forester
(Jupiterimages/Pixland/Getty Images)

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, signalling 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.

Skill level:

Things you need

  • Socket wrench
  • Razor or putty knife

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  1. 1

    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.

  2. 2

    Remove the vacuum tube electrical connection by hand. Take out the single mounting bolt located in the centre of the sensor using a socket wrench. Gently tap the sensor to break the seal.

  3. 3

    Take off the knock sensor. Scrape away the built-up debris on the engine block using a razor or putty knife.

  4. 4

    Mount the new knock sensor on the engine block. Tighten the centre bolt using the socket wrench. Reconnect the vacuum tube electrical connection.

  5. 5

    Start the engine and allow it to run for several minutes. Test-drive the vehicle, making rapid accelerations. Listen for signalling from the sensor. Check to see if the "check engine" light remains off.

Tips and warnings

  • Never disconnect the knock sensor simply to avoid the sound.

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