P0172 on a Subaru: Causes, Fixes & Repair Cost

🟠 Moderate severity · drivable

You can drive short-term, but the rich mixture wastes fuel and can damage the catalytic converter, so fix it soon.

What P0172 means on a Subaru

On a Subaru, P0172 means bank 1 is running rich — too much fuel or not enough air in the mixture. The Subaru computer has already trimmed fuel to its limit and still sees a rich condition, so it sets the code.

Is P0172 common on Subarus?

P0172 is seen on Subarus with higher mileage, often on the Outback, Forester, and Impreza. Subaru MAF sensors are a known trouble spot and a common cause of rich codes when contaminated or failing. Aging oxygen sensors and leaking fuel injectors are also common.

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

Symptoms

Check engine light on
Poor fuel economy
Strong fuel smell from the exhaust
Rough idle

Common causes

most common: Contaminated or failing MAF sensor
common: Aging or faulty oxygen sensor
common: Leaking fuel injector
less common: Faulty fuel pressure regulator holding pressure too high
less common: Restricted air filter or intake

How to fix it

Fix	DIY?	Typical cost
Clean or replace the MAF sensor	Yes	$10–300
Replace the faulty oxygen sensor	Yes	$60–200
Replace leaking fuel injectors or the pressure regulator	Mechanic	$150–500