P0171 on a 2015 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 2015 Nissan Altima

Ranked from 941 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. If it is dirty or failing, it can under-report airflow, causing the engine computer to run the mixture too lean, triggering P0171.
Reported in 2 NHTSA complaints
2
Valve Cover Gasket Leak / PCV Issues
Medium severity
A leaking valve cover gasket or PCV system issue can introduce unmetered air into the crankcase, which is drawn into the intake, causing a lean condition. Oil in spark plug tubes is a common symptom.
Reported in 2 NHTSA complaints
3
Exhaust Manifold Leak
Medium severity
An exhaust leak upstream of the primary oxygen sensor can introduce outside air into the exhaust stream, causing the O2 sensor to falsely read a lean condition and trigger P0171.
Reported in 1 NHTSA complaint
4
Vacuum Leaks (Intake Manifold Gasket, Vacuum Hoses, PCV System)
Medium severity
Unmetered air entering the engine after the MAF sensor leans out the air/fuel mixture. Listen for a hissing sound under the hood and check fuel trims at idle versus higher RPMs.
Common cause
5
Faulty Upstream Oxygen (O2) Sensor or Air/Fuel Ratio Sensor
Medium severity
A degraded or failing upstream O2 sensor may falsely report a lean condition to the ECM. Diagnosis involves monitoring sensor voltage and response time with a scan tool.
Common cause
6
Low Fuel Pressure (Failing Fuel Pump, Clogged Fuel Filter, or Bad Fuel Pressure Regulator)
High severity
If the engine is not receiving enough fuel to match the incoming air, a lean condition occurs. Check fuel pressure with a mechanical gauge to verify it meets specifications.
Common cause
7
Clogged or Dirty Fuel Injectors
Medium severity
Restricted fuel injectors will not spray the commanded amount of fuel, leading to a lean mixture in one or more cylinders. Fuel injector cleaning or replacement may be required.
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