P0171 on a 2017 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 2017 Toyota Tacoma

Ranked from 237 NHTSA owner complaints filed for this vehicle.

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1
Vacuum Leak (Intake Manifold / ACIS Flap Failure)
High severity
A broken Acoustic Control Induction System (ACIS) flap or intake manifold failure can cause unmetered air to enter the engine, leading to a lean condition (P0171). Look for physical damage to the intake manifold flaps or unmetered air leaks.
Reported in 2 NHTSA complaints
2
Faulty MAP Sensor / Throttle Body Issues
Medium severity
A failing MAP sensor or throttle body can cause incorrect air pressure readings or improper airflow, leading to lean codes and engine surging/lunging. Confirmed by checking sensor data and throttle body operation.
Reported in 1 NHTSA complaint
3
Faulty Mass Air Flow (MAF) Sensor
Medium severity
A dirty or failing MAF sensor incorrectly measures incoming air, causing the engine computer to under-fuel the engine. Confirmed by checking MAF sensor readings and cleaning/replacing the sensor.
Common cause
4
Exhaust Leak (Upstream of O2 Sensor)
Medium severity
An exhaust leak before the primary oxygen sensor introduces outside air into the exhaust stream, tricking the sensor into reading a lean condition. Confirmed by inspecting exhaust manifolds and gaskets for soot or listening for ticking noises.
Common cause
5
Low Fuel Pressure (Failing Fuel Pump or Clogged Filter)
High severity
Inadequate fuel delivery prevents the engine from maintaining the proper air/fuel ratio, resulting in a lean condition. Confirmed by testing fuel pressure at the rail under load.
Common cause
6
Faulty Oxygen (O2) or Air/Fuel Ratio Sensor
Medium severity
A degraded upstream O2 or A/F sensor may falsely report a lean condition to the PCM. Confirmed by monitoring sensor voltage sweeps and response times with a scan tool.
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