P0171 on a 2019 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 2019 Honda Civic Si

Ranked from 356 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Weak or failing fuel pump (low fuel pressure)
High severity
This generation Civic Si/Type R was subject to NHTSA recall 23V-858 for a fuel pump impeller that deforms and starves the engine of fuel pressure; many owners in this batch describe stalling, no-start, and fuel-pump grinding tied to that exact recall, which is a classic driver of a lean (P0171) fuel trim.
Reported in 24 NHTSA complaints
Parts:fuel pump
2
Failing or clogged fuel injector
Medium severity
One owner reported an outright fuel injector failure at 58,000 miles, describing it as a known supplier defect on this model; a weak injector reduces delivered fuel and pushes fuel trim lean.
Reported in 1 NHTSA complaint
Parts:fuel injector
3
Vacuum or intake air leak
Medium severity
Unmetered air entering after the MAF sensor (cracked hose, loose intake boot) is the single most common cause of a lean code and should be checked with a smoke test.
Common cause
Parts:vacuum hose
4
Dirty or failing mass airflow (MAF) sensor
Medium severity
A contaminated or drifting MAF under-reports incoming air, causing the ECU to under-fuel and trip lean trims.
Common cause
Parts:mass airflow sensor
5
Intake manifold gasket failure
Medium severity
A leaking intake manifold gasket admits unmetered air downstream of the throttle, producing a lean condition, especially at idle.
Common cause
Parts:intake manifold gasket
6
Stuck-open PCV valve
Low severity
A PCV valve stuck open draws excess crankcase air into the intake, leaning the mixture, particularly at idle.
Common cause
Parts:PCV valve
7
Exhaust leak upstream of the front oxygen sensor
Medium severity
An exhaust leak ahead of the upstream O2 sensor draws in outside air, causing the sensor to read falsely lean.
Common cause

Part links go to Amazon search results for this vehicle. As an Amazon Associate, WrenchConnect earns from qualifying purchases — always confirm fitment for your exact trim and engine before ordering.

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