P0171 on a 2019 Subaru Crosstrek

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 2019 Subaru Crosstrek

Ranked from 281 NHTSA owner complaints filed for this vehicle.

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1
Fuel pressure sensor / fuel delivery fault
Medium severity
One owner had the fuel pressure sensor replaced (along with the charcoal canister) to resolve a recurring check-engine light, indicating fuel-delivery components as a documented source of driveability/fuel-trim complaints on this vehicle.
Reported in 1 NHTSA complaint
Parts:fuel pressure sensor fuel pump
2
EVAP charcoal canister/purge assembly fault affecting fuel trim
Low severity
The same repeated-CEL case involved replacement of the charcoal canister assembly alongside the fuel pressure sensor, consistent with an EVAP-side fault disturbing the fuel trim calculation.
Reported in 1 NHTSA complaint
Parts:charcoal canister purge valve
3
Vacuum or intake leak
Medium severity
A cracked intake boot, PCV hose, or vacuum line admits unmetered air and is the most common cause of a lean code.
Common cause
4
Dirty or failing MAF sensor
Medium severity
An under-reporting mass airflow sensor causes the ECM to under-fuel relative to actual incoming air.
Common cause
Parts:mass airflow sensor
5
Weak fuel pump or clogged fuel filter
Medium severity
Reduced fuel delivery volume/pressure leans the mixture under load.
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, leaning the mixture at idle.
Common cause
Parts:PCV valve
7
Exhaust leak upstream of the front O2 sensor
Medium severity
A manifold or downpipe leak draws in outside air that the upstream sensor reads as lean.
Common cause
8
Intake manifold gasket leak
Medium severity
A failed intake gasket admits unmetered air directly into the runners, producing a 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