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P0171

P0171 on a 2016 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 2016 Subaru Outback

Ranked from 639 NHTSA owner complaints filed for this vehicle.

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  1. 1

    Exhaust leak upstream of the front O2/AFR sensor (cracked exhaust manifold/front-pipe bellows - Subaru Campaign WUJ-95)

    High severity

    This is overwhelmingly the dominant issue in the complaint set: a cracked bellows section of the front exhaust pipe/manifold (the subject of Subaru's WUJ-95 service campaign) lets outside air reach the upstream sensor, which the ECM can read as a lean condition, in addition to letting exhaust fumes into the cabin.

    Reported in 15 NHTSA complaints
    Parts:exhaust manifoldfront exhaust pipe
  2. 2

    Cracked intake manifold causing a vacuum leak

    Medium severity

    One owner's vehicle had a fractured intake manifold diagnosed alongside abnormal engine noise; a manifold crack admits unmetered air directly into the intake tract, a direct cause of a lean bank code.

    Reported in 1 NHTSA complaint
    Parts:intake manifold
  3. 3

    Dirty or failing mass airflow (MAF) sensor

    Medium severity

    A contaminated or drifting MAF under-reports incoming air, causing the ECM to run lean; clean or test against expected values.

    Common cause
    Parts:MAF sensor
  4. 4

    Vacuum/intake air leak from cracked hoses or gaskets

    Medium severity

    Unmetered air entering after the MAF is a common cause of a lean bank-1 code; smoke-test the intake and PCV system.

    Common cause
  5. 5

    Weak fuel pump or clogged fuel filter

    Medium severity

    Insufficient fuel delivery under load produces a worsening lean trim; check fuel trims and rail pressure under load.

    Common cause
    Parts:fuel pump
  6. 6

    Intake manifold gasket leak

    Medium severity

    A failed gasket admits unmetered air directly downstream of the throttle body, producing a persistent lean trim.

    Common cause
    Parts:intake manifold gasket

Part links go to Amazon search results for this vehicle. As an Amazon Associate, WrenchConnect earns from qualifying purchases — always confirm fitment for your exact trim and engine before ordering.

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

1

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.

2

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.

3

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.

4

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.

5

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