P0171 on a 2015 Toyota Corolla

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 Toyota Corolla

Ranked from 218 NHTSA owner complaints filed for this vehicle.

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1
Vacuum System Leak (Brake Booster/Lines)
High severity
A whistling sound from the engine compartment combined with erratic or soft brake pedal feel indicates a vacuum leak, which introduces unmetered air into the engine and causes a lean condition.
Reported in 2 NHTSA complaints
2
Cracked EVAP Canister / Fuel Tank Leak
Medium severity
A crack in the EVAP charcoal canister or fuel tank can introduce unmetered air into the intake manifold via the purge valve, leading to a lean code.
Reported in 1 NHTSA complaint
3
Failing Fuel Pump
High severity
A weak or failing fuel pump cannot deliver adequate fuel pressure or volume to the engine, starving it of fuel and triggering a lean condition.
Reported in 1 NHTSA complaint
4
Dirty or Faulty Mass Air Flow (MAF) Sensor
Medium severity
A contaminated MAF sensor underestimates the amount of air entering the engine, causing the PCM to inject less fuel than required.
Common cause
5
Faulty Upstream Oxygen (O2) Sensor or Air/Fuel Ratio Sensor
Medium severity
A degraded or failing Bank 1 Sensor 1 O2 sensor can falsely report a lean condition to the engine computer.
Common cause
6
Clogged or Restricted Fuel Injectors
Medium severity
Carbon buildup or debris can restrict fuel injector flow, preventing enough fuel from mixing with the incoming air.
Common cause
7
Intake Manifold Gasket Leak
Medium severity
A degraded intake manifold gasket allows unmetered air to bypass the MAF sensor and enter the combustion chamber.
Common cause
8
Faulty PCV Valve or Hose
Low severity
A stuck open PCV valve or a cracked PCV hose acts as a vacuum leak, drawing excess air into the intake manifold.
Common cause
9
Exhaust Leak Before the O2 Sensor
Medium severity
An exhaust leak near the manifold can draw ambient air into the exhaust stream, tricking the O2 sensor into reading a lean mixture.
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