P0171 on a 2011 Subaru Outback

P0171 means the engine computer is adding more fuel than normal on bank 1 because it sees too much air or not enough fuel.

Not your vehicle? See general P0171 info.

Most common causes on the 2011 Subaru Outback

Ranked from 649 NHTSA owner complaints filed for this vehicle.

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1
Idle/fuel-calibration stall condition contributing to lean trim (Subaru TSB 16-90-13R)
High severity
A large number of owners describe a low-rpm idle stalling condition on deceleration, several explicitly citing Subaru TSB 16-90-13R — the same idle air/fuel calibration issue can present as a lean trim right before the stall.
Reported in 11 NHTSA complaints
2
Head gasket failure creating an unmetered air/vacuum leak path
Medium severity
This model has a well-documented pattern of head gasket failure among these owners; a breached gasket can open a small vacuum leak path that skews the fuel trim lean in addition to causing overheating.
Reported in 5 NHTSA complaints
Parts:head gasket
3
Vacuum or intake hose leak
Medium severity
Cracked hoses or a loose intake boot add unmetered air — the textbook cause of a lean-bank code.
Common cause
4
Dirty or failing MAF sensor
Medium severity
An underreporting MAF causes the ECU to add less fuel than the engine needs; clean it and compare live data to expected values.
Common cause
Parts:mass airflow sensor
5
Weak fuel pump or clogged fuel filter
Medium severity
Insufficient fuel delivery under load produces a lean trim that worsens with RPM/load.
Common cause
Parts:fuel pump fuel filter
6
Stuck-open PCV valve
Low severity
Excess crankcase air pulled into the intake leans the mixture, especially at idle.
Common cause
Parts:PCV valve
7
Exhaust leak upstream of the front O2 sensor
Medium severity
A manifold or downpipe leak lets outside air reach the sensor, misread by the ECU as a lean condition.
Common cause
8
Intake manifold gasket leak
Medium severity
A failed gasket between the manifold and heads is a common unmetered-air path producing a persistent lean code.
Common cause
Parts:intake manifold gasket

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These causes are aggregated from NHTSA owner complaints and general repair knowledge — this is not a diagnosis. P0171 on your car still needs shop confirmation before any repair.

Common symptoms

Check engine light
Rough idle
Hesitation
Surging idle
Poor fuel economy

What to check before replacing parts

Review short- and long-term fuel trims on bank 1

Look for: High positive trims (commonly worse at idle, improving with RPM) mean the computer is adding fuel to compensate for excess air or insufficient fuel — that pattern is this code's fingerprint.

Smoke-test the intake

Look for: Smoke escaping (or a hiss you can hear) at intake boots, gaskets, or vacuum hose connections confirms unmetered air is entering after the MAF sensor, which is the most common cause of a lean code.

Inspect the PCV hoses and valve

Look for: A cracked, split, or disconnected PCV hose, or a PCV valve stuck open, draws in extra unmetered air the same way a vacuum leak does.

Clean and test the MAF sensor

Look for: Visible dirt or oil film on the sensor element, or a grams/second reading that doesn't track expected airflow for RPM, means the MAF is under-reporting air and causing the computer to under-fuel.

Verify fuel pressure

Look for: Pressure below spec (weak pump, clogged filter, failing regulator) starves the engine of fuel and produces the same lean trim pattern as a vacuum leak — rule it out before chasing leaks.

Rather have a pro handle it? Find a vetted local mechanic.

Matching symptoms

Car Runs Rough