OBD-II code · Powertrain (generic)
P0171 System Too Lean (Bank 1)
Long-term fuel trim on bank 1 is at the upper limit — the ECU is adding more fuel than baseline to maintain stoichiometric mixture but cannot reach target. Indicates unmetered air entering the intake or insufficient fuel delivery.
Is it safe to drive with P0171?
Usually safe to drive in the short term. Get it diagnosed when convenient, sooner if the car drives differently.
Common causes
- Most likelyVacuum leak in intake or PCV system
- Most likelyMAF sensor contaminated, under-reading
- PossibleFuel pressure below specification
- PossibleFuel injectors partially clogged
Symptoms
- Check engine light on
- Rough idle, possible stalling
- Hesitation or hard cold start
- Power loss under load
Repair at a glance
- Difficulty
- Medium
- DIY friendly
- Often, with basic tools
- Typical labour
- 0.5–4 h
- Emissions test
- Likely to fail
- After the fix
- A drive cycle is needed before the light stays off
- Parts involved
- Intake Manifold, Mass Air Flow Sensor, Fuel Pump, Fuel Pressure Regulator, Fuel Injector, Pcv Valve
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.