Parts for your 2018 Subaru Impreza-Oxygen sensor

2018 Subaru Impreza oxygen sensor — what it does and when to replace it

Based on technical sources — including the Subaru Factory Service Manual for the 2017–2019 Impreza (FB20 engine), Subaru genuine parts catalogues, and Denso application data — the 2018 Subaru Impreza is fitted with two exhaust sensors: an upstream wideband air–fuel ratio (A/F) sensor (Bank 1 Sensor 1) and a downstream oxygen sensor (Bank 1 Sensor 2). These are critical to the car’s closed‑loop fuel control and emissions monitoring, so an oxygen-sensor is absolutely relevant and used on this model.

On the 2018 Impreza, the front A/F sensor sits before the catalytic converter and constantly measures oxygen content so the ECU can trim fuel on the fly for smooth running, decent power, and tidy fuel economy. The rear oxygen sensor, mounted after the cat, keeps an eye on catalytic converter efficiency and flags any issues that could spike emissions. Together, they help the FB20 run clean and efficiently on Aussie and Kiwi petrol in everything from short trips to long motorway hauls.

Subaru doesn’t set a fixed replacement interval for these sensors, they’re serviced on condition. Over time, contamination (oil vapour, silicone, coolant), age, or exhaust leaks can slow response and skew readings. Telltales include poorer fuel economy, a rough idle, a lazy throttle feel, or a check engine light with codes like P0131/P0134/P0420. In typical use, many sensors last 160,000–240,000 km, but high-heat, lots of short trips, or poor fuel can shorten that.

Smart servicing for a 2018 Impreza oxygen-sensor includes visual checks for damaged wiring or cracked boots, confirming there are no exhaust leaks upstream of the cat, and scanning live data for sluggish sensor switching. When replacement’s due, go for an OE-equivalent direct-fit (often Denso for Subaru) to avoid drama with trims or readiness monitors. Warm the exhaust slightly to help removal, use an O2 sensor socket, don’t twist the loom, and follow torque specs. Most new sensors arrive with pre-coated threads — skip extra anti-seize unless the manufacturer says otherwise. After installation, clear codes and complete a proper drive cycle so the ECU relearns and sets monitors.

• Replace the failed sensor first, on high‑kilometre cars, many techs also consider renewing the mate to keep response balanced.
• Avoid “universal” splice-in sensors — they often cause headaches with the Impreza’s wideband control.

Technical references: Subaru Factory Service Manual (Engine/Diagnostics: Air–Fuel Ratio Sensor and Rear Oxygen Sensor, FB20), Subaru genuine parts catalogues for 2018 Impreza, and Denso application data confirming both sensors for this model year.

Popular questions

How many oxygen sensors does a 2018 Subaru Impreza have?

It has two: a front wideband air–fuel ratio sensor before the catalytic converter and a rear oxygen sensor after the converter. The front one manages precise fuelling, the rear one checks catalytic converter performance.

When should the oxygen sensor be replaced on a 2018 Impreza?

There’s no fixed interval from Subaru. Replace if there’s a check engine light with related codes, fuel economy drops, or the sensor tests “lazy” in live data. Many owners see replacements somewhere around 160,000–240,000 km depending on driving and fuel quality.

Can you keep driving with a faulty oxygen sensor?

Short term, usually yes — the car will default to richer maps — but fuel economy suffers and the catalytic converter can cop it if left too long. Get it diagnosed and sorted promptly to avoid bigger bills.