P0171 on a 2016 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 2016 Honda Civic

Ranked from 1,061 NHTSA owner complaints filed for this vehicle.

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1
Vacuum leak (intake manifold, PCV, or vacuum hoses)
Medium severity
A vacuum leak introduces unmetered air into the engine, causing a lean condition. This is a standard and very common cause for P0171.
Common cause
2
Faulty Mass Air Flow (MAF) sensor
Medium severity
A dirty or failing MAF sensor will under-report the amount of air entering the engine, leading the PCM to provide too little fuel, resulting in a lean code.
Common cause
3
Low fuel pressure (failing fuel pump or clogged fuel filter)
High severity
If the fuel system cannot deliver adequate pressure or volume, the engine will run lean. This is a standard diagnostic check for P0171.
Common cause
4
Faulty or clogged fuel injectors
Medium severity
Clogged injectors restrict fuel flow into the cylinders, causing a lean mixture that triggers the P0171 code.
Common cause
5
Exhaust leak before the primary oxygen sensor
Medium severity
An exhaust leak upstream of the O2 sensor can pull in outside air, causing the sensor to falsely read a lean condition.
Common cause
6
Faulty primary Oxygen (O2) Sensor or Air/Fuel Ratio Sensor
Low severity
A degraded or failing upstream O2 sensor can provide inaccurate readings to the PCM, falsely indicating a lean condition and triggering the code.
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