What are the common causes of P0171?

Vacuum leak in the intake system. Mass air flow sensor dirty or failing. Weak or failing fuel pump or clogged fuel filter. Clogged fuel injectors. Faulty oxygen sensor reporting lean incorrectly.

Smoke test the intake system to locate vacuum leaks. Clean or replace the MAF sensor and verify readings on an OBD-II scanner. Check fuel pressure and volume at the rail. Monitor fuel trim values (STFT/LTFT) with a scan tool to confirm the leak side. Inspect and clean fuel injectors if fuel delivery is the cause.

P0171

System Too Lean (Bank 1)

System: Fuel and air metering (P code)
Lamp: Check Engine Light
Category: Powertrain - Fuel/Air Metering

What it means

P0171 means System Too Lean (Bank 1).

Check these first

Vacuum leak in the intake system
Mass air flow sensor dirty or failing
Weak or failing fuel pump or clogged fuel filter
Clogged fuel injectors
Faulty oxygen sensor reporting lean incorrectly

What to do next

Smoke test the intake system to locate vacuum leaks
Clean or replace the MAF sensor and verify readings on an OBD-II scanner
Check fuel pressure and volume at the rail
Monitor fuel trim values (STFT/LTFT) with a scan tool to confirm the leak side
Inspect and clean fuel injectors if fuel delivery is the cause

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Updated

Generic SAE J2012 definition. Last updated 2026-08-30.