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.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: medium
What we see in the workshop
ON FORD ECOBOOST
A lean bank on a direct-injected EcoBoost is usually an air leak rather than a fuel problem. We pressure-test the intake and charge system and inspect the PCV system, which is a known weak point. On the 1.0 three-cylinder this code alongside coolant loss warrants a proper cooling-system pressure test.
ON ALFA ROMEO
On the 1.4 MultiAir we check the intake manifold gasket and PCV before assuming a fuel delivery fault.
Symptoms
- Check engine light on
- Rough idle, possible stalling
- Hesitation or hard cold start
- Power loss under load
Common causes
Ordered by how often each turns out to be the cause.
- Vacuum leak in intake or PCV systemCommon
- MAF sensor contaminated, under-readingCommon
- Fuel pressure below specificationPossible
- Fuel injectors partially cloggedPossible
Parts involved
- Intake Manifold
- Mass Air Flow Sensor
- Fuel Pump
- Fuel Pressure Regulator
- Fuel Injector
- PCV Valve
P0171 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.