P0171 on a 2015 Nissan Sentra

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 2015 Nissan Sentra

Ranked from 348 NHTSA owner complaints filed for this vehicle.

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1
Mass Air Flow (MAF) Sensor Failure / Dirty MAF
Medium severity
A faulty or dirty MAF sensor misreports the amount of air entering the engine, leading to a lean condition. Look for erratic idle, hesitation, and MAF-specific codes (like P0101) alongside P0171.
Reported in 3 NHTSA complaints
2
Faulty Oxygen (O2) Sensor / Air Fuel Ratio Sensor
Medium severity
A degraded upstream O2 sensor can falsely report a lean condition, forcing the ECM to add fuel, or fail to read properly. Look for sluggish sensor voltage switching or a biased lean reading.
Reported in 1 NHTSA complaint
3
Vacuum Leaks (Intake Manifold Gasket, Vacuum Hoses, PCV System)
Medium severity
Unmetered air entering the engine after the MAF sensor causes a lean mixture. Look for cracked hoses, a hissing sound under the hood, or high positive fuel trims at idle that correct at higher RPMs.
Common cause
4
Low Fuel Pressure (Failing Fuel Pump or Clogged Filter)
High severity
If the engine isn't receiving enough fuel to match the incoming air, a lean code triggers. Look for loss of power at higher speeds/loads and verify fuel pressure with a mechanical gauge.
Common cause
5
Clogged or Dirty Fuel Injectors
Medium severity
Restricted injectors cannot spray the commanded amount of fuel, causing a lean condition in one or more cylinders. Look for misfires, rough idle, and verify injector pulse/flow.
Common cause
6
Exhaust Leak Before the Upstream O2 Sensor
Medium severity
An exhaust leak pulls ambient oxygen into the exhaust stream, which the O2 sensor reads as a lean engine condition. Look for soot marks around the exhaust manifold or a ticking noise on acceleration.
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