P0171 on a 2015 Ford Fusion

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 2015 Ford Fusion

Ranked from 1,049 NHTSA owner complaints filed for this vehicle.

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1
Coolant Intrusion into Cylinders (Engine Block/Head Gasket Defect)
High severity
Coolant leaks into the combustion chamber due to a known engine block design defect, causing misfires, rough running, and eventual engine failure. Confirmed by low coolant without external leaks, white smoke from exhaust, and borescope inspection showing coolant in cylinders.
Reported in 24 NHTSA complaints
2
Vacuum Leak (Intake Manifold, PCV, or Vacuum Hoses)
Medium severity
Unmetered air enters the engine, causing a lean condition. Confirmed by inspecting hoses for cracks or using a smoke machine to find leaks.
Common cause
3
Faulty Mass Air Flow (MAF) Sensor
Medium severity
A dirty or failing MAF sensor incorrectly measures incoming air, leading to a lean fuel mixture. Confirmed by checking MAF sensor readings with a scan tool or cleaning the sensor.
Common cause
4
Low Fuel Pressure (Failing Fuel Pump or Clogged Filter)
Medium severity
Insufficient fuel delivery causes a lean condition. Confirmed by testing fuel pressure at the rail with a mechanical gauge.
Common cause
5
Faulty Oxygen (O2) Sensor (Upstream)
Medium severity
A degraded upstream O2 sensor falsely reports a lean condition or fails to adjust the mixture properly. Confirmed by monitoring O2 sensor voltage sweeps on a scan tool.
Common cause
6
Clogged or Failing Fuel Injectors
Medium severity
Restricted fuel injectors fail to spray enough fuel into the cylinders, causing a lean mixture. Confirmed by injector balance testing or observing misfires on specific cylinders.
Common cause

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