P0171 on a 2015 Subaru Forester

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 2015 Subaru Forester

Ranked from 613 NHTSA owner complaints filed for this vehicle.

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1
Faulty Mass Air Flow (MAF) Sensor
Medium severity
A dirty or failing MAF sensor misreports the amount of air entering the engine, causing the ECM to under-fuel. Confirmed by checking MAF g/s readings on a scan tool or cleaning/replacing the sensor.
Reported in 1 NHTSA complaint
2
Exhaust Leak Before the Upstream O2 Sensor
Medium severity
An exhaust leak introduces outside oxygen into the exhaust stream, tricking the O2 sensor into reading lean. Look for cracked exhaust manifolds or failed catalytic converter joints.
Reported in 1 NHTSA complaint
3
Vacuum Leak (Intake Manifold Gaskets, Vacuum Hoses, PCV)
Medium severity
Unmetered air entering the engine leans out the air/fuel mixture. Look for cracked hoses, hissing sounds, or check fuel trims while spraying carb cleaner near intake gaskets.
Common cause
4
Low Fuel Pressure (Failing Fuel Pump or Clogged Filter)
High severity
Insufficient fuel delivery prevents the engine from maintaining the proper air/fuel ratio. Confirmed by testing fuel pressure at the rail with a mechanical gauge.
Common cause
5
Faulty Upstream Oxygen (O2) Sensor or Air/Fuel Ratio Sensor
Medium severity
A degraded sensor can falsely report a lean condition or fail to respond accurately to mixture changes. Confirmed by observing sluggish or skewed sensor voltage/current data on a scan tool.
Common cause
6
Clogged or Dirty Fuel Injectors
Medium severity
Restricted injectors fail to spray the commanded amount of fuel, leading to a lean mixture in one or more cylinders. Confirmed by injector balance testing or observing misfires alongside the lean code.
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