Parts for your 2009 Subaru Impreza-Knock sensor

2009 Subaru Impreza Knock Sensor

Yes, the 2009 Subaru Impreza is fitted with a knock sensor across its EJ-series engines (2.0 and 2.5 litre, including WRX/STI). This is documented in the Subaru Factory Service Manual for the 2009MY Impreza under Engine Diagnostics (DTCs P0325–P0328: Knock Sensor Circuit), and it’s listed in Subaru’s parts catalogue as the “Knock Sensor – Engine.” Technical data providers commonly used in workshops (e.g., OEM service information, Autodata/ALLDATA) also specify a single piezoelectric knock sensor mounted on the engine block.

This knock sensor listens for detonation (knock) under the bonnet and lets the ECU dial ignition timing back just enough to keep things smooth. That means better reliability, decent fuel economy, and no nasty rattles under load. On a healthy 2009 Impreza, the sensor constantly feeds back to the ECU so it can keep timing on the sweet spot for Aussie and Kiwi fuels, whether it’s the daily commute or a spirited run up a hilly backroad.

When the knock sensor or its wiring is unhappy, the car might feel a bit doughy off the line, use more fuel than usual, or log a Check Engine Light with codes like P0325, P0326, P0327 or P0328. Some drivers also report pinging on climbs or hot days. Because the ECU goes conservative when it can’t trust the signal, performance can drop even if the engine itself is fine.

Knock sensors aren’t a routine replacement item, but they do deserve a once-over during servicing. A smart approach is to check the harness and connector for oil ingress, corrosion, or a brittle sheath, and make sure the mounting surface on the block is clean and dry. If replacement is needed, use a quality OEM-equivalent unit and torque the retaining bolt to the factory spec (about 24 N·m) — over-tightening can damage the piezo element. Fitment is straightforward for a competent home mechanic: disconnect the battery, access the sensor on the top side of the block under the intake area, swap it over, and clear fault codes. After replacement, a short road test with a scan tool watching knock correction shows the ECU is happy. Pair the fix with good fuel and regular servicing, and the Impreza’s engine will stay chirpy for heaps of kilometres without dramas.

• Common fault codes: P0325, P0326, P0327, P0328
• Typical torque: ~24 N·m (follow Subaru’s spec)
• Best practice: keep the mounting pad clean and dry

FAQs

Where is the knock sensor on a 2009 Subaru Impreza?
It’s bolted to the engine block under the intake area, roughly central on the top side of the block. Access is from above with the intake plumbing out of the way. A torch and a small socket set make life easier.

Do knock sensors need regular replacement?
No — they’re not a scheduled service item. Replace only if faulty, damaged, or when there’s verified wiring/connectivity issues and related DTCs. During routine services, a quick visual check of the connector and loom is worth doing.

Is it safe to drive with a faulty knock sensor?
The car will usually run, but the ECU may pull timing to protect the engine, which means reduced performance and higher fuel use. Persistent detonation without proper sensor feedback can risk engine damage, so it’s best to sort it promptly.