P0171 on a 2013 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 2013 Subaru Outback

Ranked from 295 NHTSA owner complaints filed for this vehicle.

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1
Clogged Catalytic Converter
Medium severity
Often a secondary failure due to excessive oil consumption, a failing catalytic converter can alter exhaust flow and sensor readings, contributing to fuel trim issues.
Reported in 3 NHTSA complaints
2
Failing Oxygen (O2) Sensor / Air Fuel Ratio Sensor
Medium severity
A degraded upstream O2/AFR sensor can falsely report a lean condition or fail due to excessive oil burning. Confirmed by sluggish sensor voltage response.
Reported in 2 NHTSA complaints
3
Faulty Mass Air Flow (MAF) Sensor
Medium severity
A dirty or failing MAF sensor incorrectly measures incoming air, causing the ECM to under-fuel the engine. Confirmed by cleaning the sensor or checking MAF g/s readings.
Reported in 1 NHTSA complaint
4
Vacuum Leaks (Intake Manifold Gaskets / Hoses)
Medium severity
Unmetered air enters the engine through degraded intake gaskets or cracked vacuum hoses, causing a lean condition. Look for hissing sounds or check fuel trims.
Common cause
5
Low Fuel Pressure (Failing Fuel Pump or Filter)
High severity
Inadequate fuel delivery prevents the engine from maintaining the correct air/fuel ratio. Confirmed by testing fuel pressure at the rail under load.
Common cause
6
Exhaust Leaks (Pre-O2 Sensor)
Medium severity
Cracks or gasket leaks in the exhaust manifold allow outside oxygen to reach the O2 sensor, tricking it into reading lean.
Common cause
7
PCV Valve Failure
Low severity
A stuck open PCV valve acts as a massive vacuum leak, drawing unmetered air and oil vapor into the intake. Common on Subaru engines.
Common cause

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