Parts for your 2009 Subaru Forester-Oxygen sensor

2009 Subaru Forester oxygen sensor: what it does and when to replace it

Yes, the 2009 Subaru Forester uses oxygen-sensing hardware. Subaru’s factory service information for the 2009MY Forester (SH) and the OEM parts catalogue specify two exhaust gas sensors on both engines offered that year: a front air–fuel ratio (A/F, wideband) sensor upstream of the catalytic converter and a rear heated oxygen (O2) sensor downstream. The diagnostic section of the service manual also includes OBD‑II checks and DTCs for these sensors (for example, P0130–P0161 and catalyst monitoring codes), confirming they’re fitted and monitored on the EJ253 (2.5i) and EJ255 (2.5XT) engines.

On this Forester, the front A/F sensor constantly reports the oxygen content of the exhaust so the ECU can fine‑tune fuel trims and keep the mixture right on the money, which helps with smooth running, good fuel economy and lower emissions. The rear O2 sensor sits after the cat and largely keeps an eye on catalytic converter efficiency. If the rear sensor sees behaviour too similar to the front one, the ECU flags it with a fault light and code, often pointing to an ageing cat or a lazy sensor.

There isn’t a strict time or kilometre interval to replace these on a 2009 Forester, but they are wear items. Many owners find sensors start to slow down somewhere around 160,000–200,000 km, or earlier if the car sees a lot of short trips. Signs to watch for include higher petrol use, rough idle, a sulphur smell, hesitations on light throttle, or the check engine light with codes related to A/F or O2 sensors or a P0420 catalyst code.

When servicing, it’s smart to inspect the wiring and plugs, look for exhaust leaks at the manifold and front pipe, and check for oil or coolant contamination that can shorten sensor life. If replacement is needed, use the correct wideband A/F sensor up front and the matching heated O2 sensor at the rear—quality OEM‑equivalent (often Denso on Subarus) parts avoid headaches. Remove them with the right O2 sensor socket, on a warm (not hot) exhaust, and refit with the supplied crush washer and thread compound if pre‑coated. Clear fuel trims and fault codes after the job and complete a proper drive cycle so the ECU can relearn. A workshop with a scan tool can confirm live sensor response and save guesswork.

• Locations: front sensor in the exhaust header/pre‑cat area, rear sensor after the catalytic converter under the floor.
• Tip: fix any exhaust leaks first—leaks can mimic a “bad sensor”.

Popular questions

How many oxygen sensors are on a 2009 Subaru Forester?

Two. There’s a front wideband air–fuel ratio sensor before the catalytic converter and a rear heated oxygen sensor after it. Both the 2.5i and 2.5XT setups use this arrangement to manage fuelling and to monitor the cat.

When should the oxygen sensors be replaced on a 2009 Forester?

They’re normally replaced when fault codes or symptoms appear rather than on a fixed schedule. Expect lifespan around 160,000–200,000 km, but city driving and short trips can shorten that. If you’re chasing poor economy or a P0420/P013x code, test first, then replace the suspect sensor with the correct type.

Can an oxygen sensor be cleaned instead of replaced?

Not reliably. Once the sensing element is contaminated or slow to respond, cleaning won’t restore proper function. The dependable fix is replacement, along with checking for causes like exhaust leaks or oil/coolant contamination.