How to Replace a Subaru Knock Sensor


Spark knock in a modern Subaru engine has become a thing of the past with the advent of the knock sensor. When the knock sensor detects a spark knock in your Subaru's engine, it sends a message to the vehicle's computer to change the ignition timing to remedy the knock. A malfunctioning spark knock detector can be very damaging to your Subaru's engine by causing the engine to detonate, or explode, at the wrong interval.

Things You'll Need

  • Metric combination wrench set
  • 3/8-inch drive ratchet wrench
  • 12 mm socket
  • Knock sensor
  • Open the hood, and disconnect the battery negative cable with a wrench.

  • Locate the black rubber air intake tube that runs from the air cleaner box to the throttle body on top of the engine.

  • Loosen the clamp on the air intake tube at the throttle body with a wrench. Grasp the tube and pull it off the throttle body inlet, and move it out of the way.

  • Locate the knock sensor. It's bolted to the engine near the throttle body. Remove the hold-down bolt attaching the knock sensor to the engine block, using a 3/8-inch drive ratchet wrench with a 12 mm socket attached.

  • Follow the electrical lead of the knock sensor to its connector, and simultaneously lift the plastic tab with your hand while gently pulling apart the plug.

  • Remove the knock sensor from the engine bay. Compare the old sensor to your new part to ensure they are identical.

  • Reattach the knock sensor to the engine block by threading in the original holding bolt by hand. Snug the bolt with the ratchet wrench and 12 mm socket. Do not over-tighten the bolt.

  • Plug the new knock sensor into the original electrical plug.

  • Gently push the throttle body air intake tube onto the throttle body. Tighten the hose clamp with a wrench.

  • Reconnect the battery's negative terminal with a wrench. Close the hood.

  • Start your Subaru's engine. Take the car on a test-drive and verify that the spark knock has disappeared.

Tips & Warnings

  • Most auto parts dealers will not accept returns of electrical parts after the parts have been installed. Ensure you have the correct part before trying to install it.

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