P0171 on a 2018 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 2018 Subaru Forester

Ranked from 489 NHTSA owner complaints filed for this vehicle.

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1
Weak or failing fuel pump
High severity
This vehicle has a cluster of stalling-in-traffic complaints explicitly attributed to the fuel pump, plus a fuel pump directly diagnosed as needing replacement — a pump that can't hold rail pressure under load leans the mixture.
Reported in 4 NHTSA complaints
Parts:fuel pump
2
Exhaust manifold gasket leak / warped, separated exhaust manifold upstream of the O2 sensor
High severity
Owners report an exhaust manifold that warped and separated from the engine (letting exhaust into the cabin) and repeated exhaust-manifold gasket leaks with a cold-start whistle — an exhaust leak ahead of the front O2 sensor draws in fresh air that reads as excess oxygen and pushes fuel trim lean.
Reported in 3 NHTSA complaints
Parts:exhaust manifold gasket exhaust manifold
3
Vacuum or intake air leak
Medium severity
Unmetered air entering after the MAF sensor is one of the most common causes of a lean (P0171) code.
Common cause
4
Dirty or failing MAF sensor
Medium severity
A contaminated or drifting MAF sensor under-reports incoming airflow, causing the PCM to under-fuel.
Common cause
Parts:mass airflow sensor
5
Stuck-open PCV valve
Low severity
A PCV valve stuck open pulls excess crankcase air into the intake, leaning the mixture at idle.
Common cause
Parts:PCV valve
6
Intake manifold gasket leak
Medium severity
A leaking manifold gasket admits unmetered air downstream of the MAF sensor, producing a persistent lean trim.
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