P0171 on a 2019 Nissan Altima

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 Nissan Altima

Ranked from 224 NHTSA owner complaints filed for this vehicle.

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1
Mass Air Flow (MAF) Sensor Failure / Dirty MAF
Medium severity
The MAF sensor measures the amount of air entering the engine. A faulty or dirty sensor will under-report airflow, causing the ECM to inject less fuel, resulting in a lean condition (P0171).
Reported in 1 NHTSA complaint
2
Vacuum Leak (Intake Manifold, PCV Hoses, or Vacuum Lines)
Medium severity
Unmetered air entering the engine after the MAF sensor throws off the air-fuel ratio, causing a lean condition. Look for cracked hoses, a loose intake manifold, or a hissing sound while the engine is running.
Common cause
3
Weak Fuel Pump / Low Fuel Pressure
High severity
If the fuel pump is failing or the fuel filter is clogged, the engine won't receive enough fuel to match the incoming air, triggering a lean code. Confirmed by testing fuel pressure at the rail.
Common cause
4
Faulty Oxygen (O2) Sensor (Upstream)
Low severity
The upstream O2 sensor monitors the oxygen content in the exhaust. If it fails or gets contaminated, it may falsely report a lean condition to the ECM.
Common cause
5
Clogged or Dirty Fuel Injectors
Medium severity
Restricted fuel injectors will not spray the commanded amount of fuel into the cylinders, leading to a lean mixture. Confirmed by injector balance testing or observing fuel trims.
Common cause
6
Exhaust Leak Before the Upstream O2 Sensor
Medium severity
An exhaust leak near the engine block can draw in outside air (oxygen) between exhaust pulses, which the O2 sensor reads as a lean condition, triggering P0171.
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