P0133 on a Subaru: Causes, Fixes & Repair Cost

🟠 Moderate severity · drivable

A Subaru with P0133 usually still drives fine, but fuel economy drops a little until the sensor is replaced.

What P0133 means on a Subaru

On a Subaru, P0133 means the bank 1 upstream oxygen sensor is switching too slowly. The front oxygen sensor on a Subaru is also the main sensor used for fuel trim, so a slow reading throws off the fuel mixture.

Is P0133 common on Subarus?

P0133 is common on Subarus like the Outback, Forester, and Impreza as they age. The front O2 sensor wears out over time, and exhaust leaks at the header or manifold are a frequent extra cause on these boxer engines.

Commonly reported on: Outback, Forester, Impreza, Legacy.

Symptoms

Check engine light on
Slightly worse fuel economy
Rough idle or hesitation
Failed emissions test

Common causes

most common: Aging upstream oxygen sensor (bank 1, sensor 1)
common: Exhaust leak at the header or manifold
common: Sensor contaminated by oil or coolant
less common: Wiring or connector damage

How to fix it

Fix	DIY?	Typical cost
Inspect the exhaust for leaks at the header before replacing the sensor	Yes	$50–150
Replace the front O2 sensor	Yes	$90–230
Repair the sensor wiring or connector	Mechanic	$100–250