P0171 on a 2018 Honda Civic Si

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 2018 Honda Civic Si

Ranked from 598 NHTSA owner complaints filed for this vehicle.

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1
Weak/failing low-pressure fuel pump module (NHTSA recall 23V-858)
High severity
An extremely large share of complaints describe the recalled fuel pump's swelling impeller cutting fuel delivery, stalling, or preventing starts; reduced fuel pressure from this failure is a textbook lean-code trigger.
Reported in 21 NHTSA complaints
Parts:fuel pump
2
Leaking fuel injector diluting oil / inconsistent delivery
Medium severity
One owner had a confirmed misfire with a fuel injector diagnosed as leaking gas into the engine oil, which also disrupts proper cylinder fueling.
Reported in 1 NHTSA complaint
Parts:fuel injector
3
EVAP/vacuum line not fully reseated during fuel pump recall service
Medium severity
One owner reported a new gasoline smell and a 2 mpg fuel-economy drop right after the fuel pump recall repair, consistent with a line not being fully reseated during the service.
Reported in 1 NHTSA complaint
4
Vacuum or intake leak
Medium severity
A cracked hose or loose intake fitting downstream of the MAF sensor admits unmetered air, leaning the mixture; smoke-test to confirm.
Common cause
Parts:intake gasket
5
Dirty or failing mass airflow (MAF) sensor
Medium severity
A contaminated or drifting MAF sensor under-reports incoming air, causing the PCM to under-fuel; check sensor readings against expected values at idle and cruise.
Common cause
Parts:MAF sensor
6
Stuck-open PCV valve
Low severity
Excess unmetered air pulled through a stuck PCV valve leans the mixture, especially noticeable at idle.
Common cause
Parts:PCV valve
7
Exhaust leak upstream of the oxygen sensor
Low severity
A leaking exhaust joint ahead of the front O2 sensor draws in outside air, biasing the sensor toward a lean reading.
Common cause

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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