P0171 on a 2015 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 2015 Toyota Tacoma
Ranked from 100 NHTSA owner complaints filed for this vehicle.
- 1
Rodent damage to wiring harness
High severitySoy-based wire insulation attracts rodents, leading to chewed wires and sensor communication failures. Look for physical damage to wiring harnesses, especially under the engine cover.
Reported in 4 NHTSA complaints - 2
Vacuum leak (Intake manifold, hoses, or PCV)
Medium severityUnmetered air entering the engine causes a lean condition. Listen for hissing sounds and check fuel trims while spraying carb cleaner around intake gaskets and vacuum lines.
Common cause - 3
Faulty Mass Air Flow (MAF) Sensor
Medium severityA dirty or failing MAF sensor under-reports the amount of air entering the engine, causing the ECM to provide too little fuel. Check MAF sensor readings in live data and inspect for dirt/debris.
Common cause - 4
Low fuel pressure (Failing fuel pump or clogged filter)
High severityInadequate fuel delivery prevents the engine from maintaining the proper air/fuel ratio. Test fuel pressure at the rail to confirm it meets manufacturer specifications.
Common cause - 5
Clogged or faulty fuel injectors
Medium severityRestricted injectors fail to spray enough fuel into the cylinders, resulting in a lean mixture. Perform an injector balance test or check for misfires on specific cylinders.
Common cause - 6
Exhaust leak upstream of the O2 sensor
Medium severityAn exhaust leak allows outside air to reach the upstream oxygen sensor, falsely indicating a lean condition. Inspect exhaust manifolds and pipes for soot marks or ticking noises.
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.
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