P0171 on a 2017 Ford Focus

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 2017 Ford Focus

Ranked from 892 NHTSA owner complaints filed for this vehicle.

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1
Failing/stuck canister purge valve (CPV), subject of Ford recall
High severity
Multiple owners report the CPV failing or being replaced under Ford recall campaigns (e.g., NHTSA 19V515000, 24N07) and causing stalling, especially with lower fuel levels; a purge valve stuck open pulls unmetered vapor/air into the intake and can drive a lean condition. Check for stored EVAP codes alongside the P0171 and test CPV operation.
Reported in 4 NHTSA complaints
Parts:purge valve
2
Weak fuel pump or failing fuel level/pressure components
Medium severity
One owner had the fuel pump and sender assembly replaced for stalling; reduced fuel delivery pressure under load can present as a lean trim.
Reported in 1 NHTSA complaint
Parts:fuel pump
3
Vacuum or intake air leak
Medium severity
A cracked hose or loose intake boot lets unmetered air past the MAF, producing a lean trim; smoke-test the intake tract.
Common cause
4
Dirty or failing MAF sensor
Medium severity
A contaminated or drifting MAF under-reports airflow, causing the PCM to under-fuel.
Common cause
Parts:MAF sensor
5
Stuck-open PCV valve
Low severity
A PCV valve stuck open draws excess crankcase air into the intake, leaning the mixture.
Common cause
Parts:PCV valve
6
Exhaust leak upstream of the upstream O2/air-fuel sensor
Low severity
A leak ahead of the sensor draws in outside air, making the sensor read falsely lean.
Common cause
7
Intake manifold gasket leak
Medium severity
A failed manifold gasket is a common vacuum-leak source and produces a lean trim; smoke-test manifold seams.
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