Parts for your 2009 Subaru Legacy-Knock sensor

2009 Subaru Legacy knock sensor — what it does and when to replace it

Yes, the 2009 Subaru Legacy uses a knock sensor. This is documented in Subaru’s Factory Service Manual for the 2005–2009 Legacy/Outback (engine diagnostics and wiring sections), the official Subaru wiring diagrams (engine control/ECM inputs), and the Subaru parts catalogue for MY2009, which lists a knock sensor for the 2.5-litre EJ engines and a pair of knock sensors for the 3.0-litre EZ engines. These technical references confirm the sensor is a standard part of the engine management system for this model year.

On a 2009 Subaru Legacy, the knock sensor’s job is simple but crucial: it listens for detonation (engine “pinging”) and lets the ECU pull timing or tweak fuelling before things get expensive. That smart correction helps the boxer engine keep its pep, keeps fuel economy tidy, and protects hardware like pistons, bearings, and head gaskets. It’s especially handy on hot days, on steeper New Zealand hills, or when the fuel isn’t top shelf.

There are a couple of different setups: most 2.5-litre EJ models run a single knock sensor mounted on top of the block under the intake, while 3.0R H6 models use two sensors (one per bank). If a sensor goes crook, the ECU will usually light the MIL and log codes such as P0325/P0328. Drivers may notice flat performance, higher fuel use, or a stubborn ping under load because the ECU is stuck in a protective timing strategy.

Replacement is straightforward with the right approach. The mating surface on the block needs to be clean and dry, and the sensor must be oriented as per the factory procedure and tightened to the specified torque in the service manual. No sealants, no over-tightening, and no twisting the pigtail. After fitting, clear the codes and take it for a short learning drive (mixed load over 10–20 kilometres) to let the ECU settle. Many owners prefer an OEM Subaru sensor, cheap copies can be noisy or unreliable. Good-quality 95/98 RON petrol in Australia and New Zealand also helps reduce knock events, letting the sensor and ECU do less firefighting.

• Common signs it’s time: MIL on with P0325/P0328, sluggish throttle, poor economy, or audible pinging under the bonnet.
• Service tip: follow the factory torque and orientation, and inspect the harness for chafing while you’re there.

FAQs

Does a 2009 Subaru Legacy have one or two knock sensors?
It depends on the engine. Most 2.5-litre four-cylinder (EJ) models have one knock sensor mounted on top of the engine block, under the intake. The 3.0R six-cylinder (EZ30) uses two knock sensors, one for each bank. Subaru’s service manual and parts catalogue for MY2009 show both configurations.

What are the symptoms of a bad knock sensor on a 2009 Legacy?
Typical giveaways include a check-engine light with codes like P0325 or P0328, dull performance, worse fuel economy, and sometimes audible pinging on hills or when it’s hot. The ECU may run safer ignition timing if it thinks the signal isn’t right, which protects the engine but makes it feel a bit flat.

Is it safe to drive with a faulty knock sensor?
Short trips to a workshop are usually fine, but it’s not ideal to leave it. With a faulty sensor the ECU can’t accurately react to knock, and prolonged detonation can damage the engine. Get it diagnosed, check the wiring, and replace the sensor if needed, then clear the codes and complete a learning drive.