SUBARU knock sensor - what is it?

The Subaru knock sensor is a device that outputs a signal that is read by the ECU (engine control unit) to determine if engine knock is present. If the ECU believes it is detecting engine knock, it then retards the ignition timing until knock is no longer detected.

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Which SUBARU models are equipped with knock sensors?

SUBARU models starting around the early 1990's have knock sensors. Typically any vehicle that has an ECU (engine control unit) utilizes a knock sensor. Vehicles that have a distributor / vacuum advance timing system do not typically have a knock sensor.

What does a SUBARU knock sensor look like?

A typical SUBARU knock sensor photo is at the right. The style has changed a few times, mainly in the connector.

Where is the SUBARU knock sensor located?

The typical location on a SUBARU vehicle is on the engine block somewhat near the throttle body. The photo at the right shows the typical SUBARU knock sensor location. On H6 engines the location is somewhat different.

How to remove or replace a SUBARU knock sensor?

Knock sensor removal is fairly straightforward on the normally aspirated 1.8L, 2.2L and 2.5L. The service manual says to remove a bunch of parts including the intake manifold, but it is really not necessary. With a flexible socket extension the knock sensor is fairly simple to remove. A 3/8" drive setup works well.

What problems can a faulty SUBARU knock sensor cause?

There are two main types of issues the knock sensor can encounter:

Problems that cause the MIL/CEL (malfunction indicator lamp/check engine light) to turn on; and,Problems that do not cause the MIL/CEL to illuminate.

We will look at these issues in more detail below.

SUBARU knock sensor problems that cause the MIL/CEL to illuminate

Generally codes P0325, P0326, P0327, and/or P0328 may be present.

P0325 Knock Sensor Circuit MalfunctionP0326 Knock Sensor Circuit Performance (might not be applicable to Subaru's).P0327 Knock sensor circuit low inputP0328 Knock sensor circuit high input

These codes mean the ECU has detected an electrical issue with the knock sensor circuit. It does not mean the ECU is detecting knock.

Problems that do not cause the MIL/CEL to illuminate

The ECU can only detect an electrical issue with the knock sensor circuit. The circuit may look ok to the ECU, but if the knock sensor itself is outputting faulty information that is still an issue.

For example, the output waveform from the knock sensor may look as if knock is occuring when in reality it is not. The result is the ECU retards timing. And retarded timing equals loss of power and fuel economy.

Cracked Subaru knock sensor

The Subaru knock sensor, especially from mid/late 90's soobs, can develop a crack. It may or may not result in the MIL/CEL to come on. At the right is an image of a cracked Subaru knock sensor.

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If you have any questions, please feel free to post a comment below, or post in the Subaru repair forum.