Parts for your 2017 Subaru Legacy-Knock sensor

2017 Subaru Legacy knock sensor—what it does and when to replace it

Yes, the 2017 Subaru Legacy is fitted with a knock sensor. Factory technical references including the Subaru Legacy/Outback 2015–2019 Service Manual (Engine Diagnostics), Subaru’s Technical Information System (STIS), and the OEM parts catalogue list a piezoelectric knock sensor and related diagnostics such as DTCs P0325, P0327 and P0328. That makes the knock sensor both relevant and essential to how the ECU manages spark timing on the FB25 2.5-litre and EZ36 3.6-litre engines.

On this model, the knock sensor’s job is to “listen” for abnormal combustion—detonation or pinging—by feeling vibration in the engine block. When it detects knock, it signals the ECU to trim ignition timing and protect the engine from damage. It’s a quiet overachiever, helping the car run smoothly on everyday Aussie and Kiwi fuels and conditions, from city commuting to long open-road runs.

There’s no scheduled replacement interval for the knock sensor, it’s generally maintenance-free. During regular servicing, it’s smart to give its wiring and connector a quick look—check for hardened insulation, corrosion or oil contamination under the bonnet. Keeping the engine in good nick also helps the knock system do its job: use quality petrol of the recommended octane, fix vacuum leaks, maintain cooling performance, and address carbon build-up if it’s suspected.

Common signs the sensor or its circuit isn’t happy include a check engine light with codes like P0325/P0327/P0328, noticeably dull performance (the ECU may run conservative timing), and higher fuel use. Occasional audible pinging under load can also crop up if knock control isn’t effective.

Replacement is straightforward for a competent home mechanic but precision matters. The sensor mounts to the engine block, access varies by engine and may require removing intake components. Always:

• Disconnect the battery negative before unplugging the sensor.
• Use a quality OEM-equivalent sensor and ensure the correct orientation on the block.
• Tighten to the factory torque spec with a torque wrench—over- or under-tightening can skew readings.
• Inspect and, if needed, replace the harness or connector pigtail, clear DTCs and perform a proper road test.

Left unfixed, a faulty knock sensor won’t usually strand the driver, but running on permanently retarded timing is no fun and can mask genuine knock risks. Sorting it promptly keeps the Legacy feeling crisp and protects the engine over the long haul.

Does a 2017 Subaru Legacy have a knock sensor?

It does. Subaru’s factory service manual and parts catalogue for the 2015–2019 Legacy/Outback platform specify a knock sensor and provide diagnostic procedures for related fault codes (P0325, P0327, P0328). Both the 2.5-litre and 3.6-litre engines rely on it for knock control.

What are the symptoms of a bad knock sensor on a 2017 Legacy?

Typical signs include a check engine light with knock-sensor codes, noticeably reduced power as the ECU pulls timing, worse fuel economy, and sometimes light pinging under load. Wiring damage or a poor connector can cause the same behaviour as a failed sensor.

Should the knock sensor be replaced as part of routine servicing?

There’s no routine replacement interval. It’s replaced when faulty or if the wiring is compromised. During major services, a quick visual check of the sensor area and harness is worthwhile, especially if the vehicle sees lots of kilometres or harsh conditions.