P0171 on a 2017 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 2017 Subaru Forester

Ranked from 578 NHTSA owner complaints filed for this vehicle.

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1
Cracked or leaking exhaust manifold/flex pipe
High severity
Look for exhaust fumes in the cabin, loud exhaust noise, or visible cracks on the exhaust manifold bellows/flex pipe. This introduces unmetered air upstream of the O2 sensor, causing a lean condition.
Reported in 5 NHTSA complaints
2
Failed or rusted catalytic converter
Medium severity
Check for loud rattling, exhaust smell, or severe rust on the catalytic converter assembly. While typically associated with P0420, severe exhaust leaks at the converter can trigger lean codes.
Reported in 4 NHTSA complaints
3
Vacuum leak (Intake manifold gaskets, vacuum hoses, PCV system)
Medium severity
Inspect all vacuum hoses, intake boots, and gaskets for cracks or disconnections. Unmetered air entering the intake manifold is the most common generic cause of a P0171 lean code.
Common cause
4
Faulty Mass Air Flow (MAF) sensor
Medium severity
Check MAF sensor readings via scan tool; a dirty or failing MAF will under-report airflow, causing the ECM to provide too little fuel. Cleaning or replacing the sensor often resolves this.
Common cause
5
Low fuel pressure (Failing fuel pump or clogged filter)
Medium severity
Test fuel pressure at the rail. If the engine is not receiving enough fuel due to a weak pump or restriction, it will run lean and trigger P0171.
Common cause
6
Faulty Air/Fuel Ratio (O2) Sensor (Upstream)
Low severity
Monitor the upstream O2 sensor voltage/current. A degraded sensor can falsely report a lean condition, prompting the ECM to set the 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