P0171 System Too Lean Bank 1

P0171 means the control module reports a lean condition on bank 1. The useful starting point is fuel-trim data: the module is adding fuel because measured combustion appears too lean.

Code details

Generic OBD-II · P0

System: Fuel / Air
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not treat P0171 as an oxygen-sensor replacement instruction. The sensor may simply be reporting a real lean condition.

Symptoms of P0171

Rough or lopey idle, sometimes stalling.
Hesitation or lack of power under acceleration.
Check-engine light with elevated long-term fuel trims.
Occasional light surging at steady cruise.

Common causes of P0171

Unmetered air — a vacuum leak, split intake boot, or PCV hose.
A dirty or failing mass-airflow (MAF) sensor.
Low fuel pressure or weak/clogged injectors.
An exhaust leak near the upstream oxygen sensor.

Useful checks before replacing parts

Review short-term and long-term fuel trims at idle, cruise, and light load.
Check for unmetered air, intake leaks, exhaust leaks ahead of oxygen sensors, weak fuel delivery, and contaminated airflow-sensor data.
Use freeze-frame data to see whether the fault appeared at idle, under load, or during a transient condition.

Related OBD-II codes

These nearby fuel / air entries can help compare wording and pin down the fault area.