P0171 on a 2021 Ford Bronco Sport

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 2021 Ford Bronco Sport

Ranked from 665 NHTSA owner complaints filed for this vehicle.

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1
Cracked/leaking fuel injector (NHTSA recall 25V467) reducing cylinder fueling
High severity
This recall-affected fleet shows repeated fuel-injector cracking that leaks fuel onto the engine instead of into the cylinder, reducing delivered fuel and producing stumble/sputter on acceleration consistent with a lean condition; one owner's scan explicitly pulled P0171 while chasing this recall.
Reported in 16 NHTSA complaints
Parts:fuel injector
2
EVAP purge valve stuck open
Medium severity
One owner's independent shop found the purge valve stuck open — a known Bronco Sport issue — pulling unmetered air into the intake and biasing fuel trims lean.
Reported in 1 NHTSA complaint
Parts:purge valve
3
Vacuum or intake leaks
Medium severity
Cracked intake boots, PCV hoses, or brake-booster lines let unmetered air past the MAF, a textbook P0171 cause.
Common cause
4
Dirty or failing MAF sensor
Low severity
A contaminated or drifting MAF under-reports intake air, causing the PCM to under-fuel and trip lean trims.
Common cause
Parts:MAF sensor
5
Weak fuel pump or clogged fuel filter
Medium severity
Insufficient fuel delivery under load produces the same lean-trim signature as an air leak.
Common cause
Parts:fuel pump fuel filter
6
Stuck-open PCV valve / faulty oil separator
Medium severity
This EcoBoost has a documented oil-separator/PCV defect; a stuck-open path adds unmetered air, especially noticeable at idle.
Common cause
Parts:PCV valve oil separator
7
Exhaust leak upstream of the O2 sensor
Low severity
Outside air drawn past a leaking exhaust joint ahead of the front O2 sensor skews the reading lean.
Common cause
8
Intake manifold gasket leak
Medium severity
A leaking manifold gasket admits unmetered air directly downstream of the throttle body.
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