P0174 on a Subaru: Causes, Fixes & Repair Cost

🟠 Moderate severity · drivable

Drivable short-term, but a lean condition on bank 2 should be diagnosed soon to avoid drivability issues.

What P0174 means on a Subaru

On a Subaru, P0174 means the engine is running lean on bank 2 — the side opposite cylinder 1 — because too much air or too little fuel is getting in. The computer has added as much fuel as it can and still can't balance the mixture. It's the bank 2 version of P0171.

Is P0174 common on Subarus?

P0174 is most common on Subaru's 6-cylinder models like the Outback and Legacy with the 3.6R engine, since only 6-cylinder Subarus have a bank 2. Vacuum leaks from cracked hoses and intake gaskets are the usual causes on these engines. A dirty MAF sensor is also a frequent trigger on higher-mileage Subarus.

Commonly reported on: Outback, Legacy, Tribeca.

Symptoms

Check engine light is on
Rough or surging idle
Hesitation or lack of power
Poor fuel economy

Common causes

most common: Vacuum leak (cracked hose or intake gasket)
common: Dirty or failing mass air flow (MAF) sensor
common: Intake air leak after the MAF sensor
less common: Faulty upstream oxygen sensor (bank 2)
less common: Weak fuel pump or clogged fuel filter

How to fix it

Fix	DIY?	Typical cost
Inspect and repair vacuum leaks (hoses, gaskets)	Yes	$10–100
Clean the mass air flow sensor with MAF cleaner spray	Yes	$10–20
Replace the bank 2 upstream oxygen sensor if it's reading wrong	Yes	$60–200
Have a smoke test performed to find the exact leak	Mechanic	$50–150

Frequently asked questions

Why is P0174 common on Subaru 6-cylinder engines?

The rubber vacuum hoses on Subaru's 6-cylinder engines can crack with age, letting unmetered air in. A small leak on bank 2 is enough to set the code.

What does bank 2 mean on a Subaru?

Bank 2 is the side of the engine that does not contain cylinder 1. Only Subaru's 6-cylinder models like the Outback 3.6R have a bank 2.

Can I keep driving my Subaru with P0174?

Short trips are fine, but a lean condition can cause rough idle and hesitation over time. Have it diagnosed soon so a small leak doesn't turn into a bigger repair.