P0171 on a 2018 Subaru Outback

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 Outback

Ranked from 805 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Weak or failing fuel pump (low fuel pressure)
Medium severity
Insufficient fuel delivery under load causes a lean condition; one owner had a stall diagnosed as a faulty fuel pump.
Reported in 1 NHTSA complaint
Parts:fuel pump
2
Leaking fuel delivery line reducing rail pressure
High severity
A ruptured/leaking fuel supply line upstream of the injectors starves the rail and can trigger lean codes; documented on this generation as a rare but real failure.
Reported in 1 NHTSA complaint
Parts:fuel line fuel injector
3
Vacuum or intake air leak
Medium severity
Unmetered air entering after the MAF causes a lean bank-1 reading; look for cracked intake boots, PCV hoses, or brake booster lines.
Common cause
4
Dirty or failing MAF sensor
Medium severity
A contaminated or drifting MAF under-reports incoming air, causing the ECU to under-fuel; clean/test the sensor and check the trim tables.
Common cause
5
Stuck-open PCV valve
Low severity
An open PCV valve draws excess uncontrolled air into the intake, especially at idle, pushing the mixture lean.
Common cause
6
Intake manifold or throttle body gasket leak
Medium severity
A worn manifold gasket lets in unmetered air; look for a hissing sound and check with smoke test.
Common cause
7
Exhaust leak upstream of the front O2 sensor
Medium severity
Outside air drawn in through an exhaust leak ahead of the sensor skews the reading lean; check manifold and flex-joint welds.
Common cause

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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