P0171 on a 2019 Toyota Tacoma

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 Toyota Tacoma

Ranked from 210 NHTSA owner complaints filed for this vehicle.

Sort by:

1
Low-Pressure Fuel Pump Failure (Recall 20V682000 / 20V012000)
High severity
A defective low-pressure fuel pump can cause severe lean running conditions, engine stalling, and elevated combustion temperatures. This is a known recall issue for this vehicle that starves the engine of fuel, directly triggering a lean code.
Reported in 5 NHTSA complaints
2
Fuel Injector Failure or Clogging
High severity
Failed or clogged fuel injectors restrict fuel delivery to the cylinders, causing a localized or bank-wide lean condition. Diagnostics often pinpoint specific cylinder misfires alongside the lean code.
Reported in 2 NHTSA complaints
3
Vacuum Leaks (Intake Manifold or Hoses)
Medium severity
Unmetered air entering the engine after the MAF sensor throws off the air-fuel ratio, causing a lean condition. Look for cracked vacuum hoses, loose intake clamps, or a failing intake manifold gasket.
Common cause
4
Dirty or Faulty Mass Air Flow (MAF) Sensor
Low severity
A contaminated MAF sensor under-reports the amount of air entering the engine, causing the ECM to inject less fuel than needed. Cleaning the sensor wire often resolves the issue if it hasn't completely failed.
Common cause
5
Faulty Upstream Air-Fuel Ratio (O2) Sensor
Medium severity
The upstream sensor monitors the oxygen content in the exhaust to adjust fuel trim. If it degrades or fails, it can falsely report a lean condition or fail to correct a real one.
Common cause
6
Exhaust Leak Before the O2 Sensor
Medium severity
An exhaust leak near the manifold can draw ambient air into the exhaust stream between exhaust pulses. The upstream O2 sensor reads this extra oxygen and falsely triggers a lean code.
Common cause
7
Faulty PCV Valve or Hose
Low severity
A stuck open PCV valve or a cracked PCV hose acts as a massive vacuum leak, introducing excess air into the intake manifold. This skews the air-fuel ratio lean.
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