P0171 on a 2015 Honda Accord

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 Honda Accord

Ranked from 594 NHTSA owner complaints filed for this vehicle.

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1
Vacuum leak (Intake manifold plenum deterioration)
Medium severity
A deteriorated intake manifold plenum causes unmetered air to enter the engine, leading to a lean condition (P0171). Look for cracks, corrosion, or hissing sounds near the intake manifold.
Reported in 1 NHTSA complaint
2
Faulty fuel pump
High severity
A failing fuel pump delivers insufficient fuel pressure, causing the engine to run lean. Confirm by testing fuel pressure at the rail.
Reported in 1 NHTSA complaint
3
Vacuum leak (General)
Medium severity
Unmetered air entering the intake tract past the MAF sensor causes a lean mixture. Look for cracked, disconnected, or degraded vacuum hoses.
Common cause
4
Dirty or faulty Mass Air Flow (MAF) sensor
Low severity
A contaminated MAF sensor under-reports airflow, causing the PCM to inject less fuel than needed. Confirm by checking MAF g/s readings on a scan tool.
Common cause
5
Clogged fuel injectors
Medium severity
Restricted injectors cannot deliver the commanded amount of fuel, resulting in a lean condition. Confirm via injector balance test or observing fuel trims.
Common cause
6
Exhaust leak before the upstream O2 sensor
Medium severity
An exhaust leak allows oxygen to enter the exhaust stream, tricking the O2 sensor into reading lean. Look for soot marks or listen for ticking noises near the exhaust manifold.
Common cause
7
Faulty upstream Oxygen (O2) or Air/Fuel Ratio sensor
Low severity
A degraded sensor may falsely report a lean condition, causing the PCM to set P0171. Confirm by graphing sensor voltage/current response.
Common cause
8
PCV valve stuck open or leaking PCV hose
Low severity
Excessive crankcase vapors or unmetered air drawn through a faulty PCV system causes a lean mixture. Inspect the PCV valve and hoses for integrity.
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.

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Matching symptoms

Car Runs Rough