P0171 – System Too Lean (Bank 1)

Description

The ECM detects a lean air-fuel mixture on Bank 1 based on fuel trim and O2 sensor feedback. Use the Fuel Trims guide and OBD2 Live Data guide to interpret the correction before replacing parts.

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Symptoms

Rough idle
Hesitation on acceleration
Lack of power
Poor fuel economy
Possible misfires

Causes

Vacuum leak at intake or hoses
Dirty or faulty MAF sensor
Weak fuel pump or clogged filter
Leaking exhaust upstream of O2 sensor

Diagnosis

Monitor short- and long-term fuel trims
Smoke-test intake for vacuum leaks
Check MAF readings vs RPM
Measure fuel pressure under load

Repair

Repair vacuum or intake leaks
Clean or replace MAF sensor
Repair fuel delivery issues
Fix exhaust leaks before O2 sensor