P0171 on a 2019 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 2019 Honda Civic

Ranked from 348 NHTSA owner complaints filed for this vehicle.

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1
Fuel Pump Failure / Improperly Molded Impeller
High severity
The fuel pump impeller can deform and cause the fuel pump to become inoperable, leading to stalling, lack of power, or a no-start condition. This is a known recall issue for this vehicle.
Reported in 3 NHTSA complaints
2
Oil Dilution / Gasoline Mixing with Engine Oil
High severity
Gasoline mixes with the engine oil, causing a strong fuel smell, potential loss of power, and limp mode. Check the oil dipstick for a strong fuel odor and elevated oil level.
Reported in 2 NHTSA complaints
3
Water Intrusion into Spark Plug Cylinders
Medium severity
Water enters from the hood scoop and drips onto the cylinders, causing the ignition coils and spark plugs to rust and misfire, triggering a check engine light.
Reported in 1 NHTSA complaint
4
Rodent Damage to Soy-Based Wiring Harness
Medium severity
Rodents chew on the soy-based engine wiring harness, causing electrical shorts, sensor failures, and check engine lights. Inspect the wiring harness for bite marks or exposed wires.
Reported in 1 NHTSA complaint
5
Faulty Mass Air Flow (MAF) Sensor
Medium severity
A dirty or failing MAF sensor incorrectly measures incoming air, causing the engine to run lean. Check MAF sensor readings and inspect for contamination.
Common cause
6
Vacuum Leaks
Medium severity
Unmetered air enters the intake manifold through cracked hoses or faulty gaskets, causing a lean condition. Perform a smoke test to locate leaks.
Common cause
7
Faulty Oxygen (O2) Sensor
Medium severity
An upstream O2 sensor may falsely report a lean condition due to degradation or contamination. Monitor O2 sensor voltage sweeps with a scan tool.
Common cause
8
Clogged Fuel Injectors
Medium severity
Restricted fuel injectors fail to deliver the commanded amount of fuel, resulting in a lean mixture. Perform an injector balance test or check for misfires.
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