P0171 on a 2019 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 2019 Honda Accord

Ranked from 650 NHTSA owner complaints filed for this vehicle.

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1
Blown Head Gasket / Coolant Intrusion
High severity
Coolant leaks into the cylinders (often cylinder 3), causing misfires, rough idle, overheating, and fuel trim issues. Confirmed by coolant loss without external leaks, white smoke from exhaust, or borescope inspection showing coolant in cylinders.
Reported in 14 NHTSA complaints
2
Faulty Fuel Injectors
Medium severity
Injectors fail or become damaged (sometimes linked to fuel pump issues or high compression from head gasket leaks), leading to misfires, rough running, and lean codes. Confirmed by misfire codes (e.g., P0303) and injector circuit codes.
Reported in 6 NHTSA complaints
3
Defective Low-Pressure Fuel Pump
High severity
A known recall issue where the fuel pump fails, causing fuel starvation, engine stalling, and lean conditions. Confirmed by loss of power, stalling, and fuel pressure testing.
Reported in 4 NHTSA complaints
4
Turbocharger Failure
High severity
Turbo failure can cause loss of power, limp mode, and air/fuel mixture issues. Confirmed by physical inspection of the turbo, underboost codes, or dealer diagnosis.
Reported in 3 NHTSA complaints
5
Vacuum Leak (Intake Manifold / Hoses)
Medium severity
Unmetered air enters the engine, causing a lean condition. Confirmed by smoke testing the intake system or listening for hissing noises.
Common cause
6
Faulty Mass Air Flow (MAF) Sensor
Low severity
A dirty or failing MAF sensor incorrectly measures incoming air, leading the PCM to under-fuel the engine. Confirmed by checking MAF sensor data on a scan tool or cleaning the sensor.
Common cause
7
Faulty Oxygen (O2) Sensor
Low severity
An upstream O2 sensor reading falsely lean will trigger P0171. Confirmed by monitoring O2 sensor voltage and response time with a scan tool.
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