P0171 on a 2017 Honda Civic

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 Honda Civic

Ranked from 574 NHTSA owner complaints filed for this vehicle.

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1
Oil Dilution (Fuel mixing with oil)
High severity
Fuel leaking into the engine oil can cause poor performance, strong fuel odors, and potential engine damage. Check oil level (high on dipstick) and smell for gasoline.
Reported in 6 NHTSA complaints
2
Faulty Fuel Injectors
High severity
Fuel injectors can fail or leak, causing misfires, poor engine performance, and lean/rich conditions. Look for misfire codes, rough running, and check engine lights.
Reported in 4 NHTSA complaints
3
Vacuum Leak (Intake Manifold/Hoses)
Medium severity
Unmetered air entering the engine causes a lean condition. Inspect intake hoses, PCV system, and manifold gaskets for cracks or leaks.
Common cause
4
Faulty Mass Air Flow (MAF) Sensor
Medium severity
A dirty or failing MAF sensor incorrectly measures incoming air, leading to a lean fuel mixture. Check MAF sensor readings and clean or replace if necessary.
Common cause
5
Low Fuel Pressure (Failing Fuel Pump/Filter)
Medium severity
Insufficient fuel delivery causes a lean condition. Test fuel pressure at the rail to ensure the pump is operating within specifications.
Common cause
6
Faulty Oxygen (O2) Sensor (Upstream)
Medium severity
A degraded primary O2 sensor can falsely report a lean condition, causing the ECU to adjust fuel trims incorrectly. Monitor O2 sensor voltage and response time.
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