P0171 on a Subaru: Causes, Fixes & Repair Cost

🟠 Moderate severity · drivable

A Subaru with P0171 is drivable short-term, but a lean mixture can cause misfires and should be fixed soon.

What P0171 means on a Subaru

On a Subaru, P0171 means bank 1 is running lean — too much air or not enough fuel — and the engine computer can't add enough fuel to fix it. Subaru's computer uses the MAF sensor and oxygen sensors, and a lean mixture past the correction limit sets P0171.

Is P0171 common on Subarus?

P0171 is common on Outbacks, Foresters, and Imprezas with higher mileage. Subaru boxer engines are prone to vacuum leaks from the intake and PCV systems, and a dirty MAF sensor is another frequent cause.

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

Symptoms

Check engine light on
Rough idle
Hesitation under acceleration
Worse fuel economy

Common causes

most common: Vacuum leak (intake boot, hoses)
common: Dirty or failing MAF sensor
common: Worn PCV valve or hose
less common: Weak fuel pump or clogged fuel filter

How to fix it

Fix	DIY?	Typical cost
Inspect and repair vacuum leaks in the intake system	Yes	$10–100
Clean the MAF sensor with MAF cleaner	Yes	$10–20
Replace the PCV valve and hose	Yes	$15–60
Replace a weak fuel pump or clogged fuel filter	Mechanic	$300–700

Frequently asked questions

Can I drive my Subaru with a P0171 code?

Yes for a short time, but a lean engine runs hotter and can misfire. Fix it soon to avoid damaging the catalytic converter or engine.

What causes P0171 on Subarus most often?

A vacuum leak in the intake boot or hoses is the most common cause, with a dirty MAF sensor close behind. A worn PCV valve can also let unmetered air in.

How much does it cost to fix P0171 on a Subaru?

Usually $10–$100 for vacuum repairs or MAF cleaning. A fuel pump failure adds $300–$700.