P0171 on a 2020 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 2020 Subaru Forester

Ranked from 593 NHTSA owner complaints filed for this vehicle.

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1
Auto Stop-Start restart stumble/stall
Medium severity
A large cluster of complaints describe stalling or failing to restart specifically when resuming from the auto idle-stop system, consistent with a lean/rough restart condition.
Reported in 14 NHTSA complaints
2
Thermostatically-controlled coolant/bypass valve (TCV) failure skewing fuel trim
High severity
A widely reported Subaru TCV defect disrupts the engine coolant temperature signal used to compute fuel trim; several complaints cite the exact fault DTCs (P2682/P26A) alongside stalling/rough-running, consistent with the ECM commanding a lean mixture off a bad temperature reading.
Reported in 10 NHTSA complaints
Parts:thermostatic control valve engine coolant bypass valve
3
Vacuum or intake air leak
Medium severity
A leak after the MAF (hose, gasket, PCV line) is the most common cause of a lean bank-1 code and is the first thing to smoke-test.
Common cause
4
Dirty or failing MAF sensor
Medium severity
A contaminated or drifting mass airflow sensor under-reports incoming air, causing the ECM to under-fuel.
Common cause
Parts:mass airflow sensor
5
Weak fuel pump or clogged fuel filter
Medium severity
Insufficient fuel pressure/volume under load produces a lean condition.
Common cause
Parts:fuel pump fuel filter
6
Stuck-open PCV valve
Low severity
A PCV valve stuck open draws excess unmetered air into the intake, especially at idle.
Common cause
Parts:PCV valve
7
Intake manifold gasket leak
Low severity
A worn gasket lets in unmetered air, confirmed with a smoke test.
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