P0171 on a 2020 Subaru Ascent

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 2020 Subaru Ascent

Ranked from 317 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Weak or failing low-pressure fuel pump (Subaru recall 21V-587000-adjacent symptoms)
High severity
Three complaints describe stalling/hesitation on acceleration suspected by owners and mechanics to be the fuel pump, explicitly tied to Subaru's low-pressure fuel pump recall population for this platform.
Reported in 3 NHTSA complaints
Parts:fuel pump
2
Stuck-open or failed PCV valve
High severity
One complaint describes a PCV valve failure that led to a catastrophic engine failure on the highway — a failed PCV can also introduce enough unmetered air to lean out the mixture before it fails completely.
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
Clogged fuel filter restricting flow
Medium severity
A plugged filter limits fuel volume under load, producing a lean trim at higher RPM; check fuel pressure/flow under load.
Common cause
Parts:fuel filter
6
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
7
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