P0171 on a 2016 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 2016 Subaru Forester

Ranked from 405 NHTSA owner complaints filed for this vehicle.

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1
Weak or failing fuel pump
High severity
Two complaints describe sudden highway stalls that owners tie to Subaru Forester fuel-pump defects/lawsuits affecting this platform generation, suspecting the same failure mode extends to this model year.
Reported in 2 NHTSA complaints
Parts:fuel pump
2
Stuck-open PCV valve allowing oil/air intrusion
Medium severity
One complaint describes a PCV valve that fails to regulate the combustion system properly, letting oil into the intake and causing power loss consistent with a lean-running condition.
Reported in 1 NHTSA complaint
Parts:PCV valve
3
Vacuum or intake air leak (hoses, intake boot, brake booster line)
Low severity
Unmetered air entering after the MAF leans the mixture; smoke-test the intake tract to find the leak.
Common cause
4
Dirty or failing MAF sensor
Low severity
An under-reporting MAF tells the PCM less air is entering than actually is, causing it to under-fuel and trim lean; clean the sensor and compare live MAF grams/sec to expected values.
Common cause
Parts:mass airflow sensor
5
Exhaust leak upstream of the air/fuel sensor
Low severity
Outside air drawn in through a manifold or pipe leak ahead of the sensor makes the PCM read lean even when the mixture is correct; inspect for soot streaking at joints.
Common cause
6
Intake manifold gasket leak
Medium severity
A leaking manifold gasket lets in unmetered air directly at the ports, often affecting only certain cylinders; smoke-test the manifold.
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