P0171 on a 2021 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 2021 Honda Accord

Ranked from 217 NHTSA owner complaints filed for this vehicle.

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1
Weak/failing fuel pump or low fuel-rail pressure (Honda low-pressure fuel pump issue)
High severity
Multiple owners report a diagnosed fuel pressure regulator/fuel pump fault, a directly-read low fuel-rail-pressure code (P0087), and stalling/limp mode tied to a fuel-pump recall pattern on this platform — a textbook cause of a system-lean code.
Reported in 5 NHTSA complaints
Parts:fuel pump fuel pressure regulator
2
Clogged or underperforming fuel injector reducing fuel delivery
Medium severity
Owners report injector replacement and a dealer-diagnosed faulty fuel injector causing hard idle/check-engine codes on this platform; reduced injector flow leans the affected cylinder(s).
Reported in 2 NHTSA complaints
Parts:fuel injector
3
Vacuum or intake leak (hose, gasket, or PCV system)
Medium severity
Unmetered air entering after the MAF sensor leans the mixture; check with a smoke test focused on intake boots, brake booster hose, and PCV lines.
Common cause
Parts:vacuum hose intake gasket
4
Dirty or failing MAF sensor
Medium severity
A contaminated or drifting MAF under-reports incoming air, so the PCM under-fuels; cleaning or swapping the sensor is the standard test.
Common cause
Parts:mass air flow sensor
5
Stuck-open PCV valve
Low severity
A PCV valve stuck open pulls excess crankcase air into the intake, leaning the mixture at idle and light cruise.
Common cause
Parts:PCV valve
6
Exhaust leak upstream of the air/fuel or O2 sensor
Medium severity
A leaking manifold gasket or cracked header lets outside air reach the sensor, making the PCM think the engine is running leaner than it is.
Common cause
Parts:exhaust manifold gasket
7
Intake manifold gasket failure
Medium severity
A failed manifold gasket is a large, direct vacuum leak and one of the more common lean-code causes on higher-mileage engines.
Common cause
Parts:intake manifold gasket

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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