P0171 – System too lean – bank 1
P0171 describes a lean fuel-control condition on bank 1. The engine monitor finds that the air-fuel relationship cannot be maintained within the intended control range.
Powertrain · Air-fuel control
Open this code in the decoderMeaning and affected system
Bank 1 contains cylinder 1. Short- and long-term fuel trims show how the controller responds to combustion measurements. The code alone cannot distinguish additional air, inadequate fuel delivery or an input affecting the calculation.
Possible causes
- Unmetered air at an intake or crankcase-ventilation connection is one possibility.
- A contaminated or inaccurate mass airflow sensor can influence mixture calculations.
- Fuel delivery and other input readings need checking for the particular engine.
Possible symptoms
- The check engine light may illuminate.
- Rough idle or misfires may accompany the fault; apparently normal running does not rule out the recorded condition.
Useful diagnostic considerations
- Compare fuel trims under different operating conditions rather than relying on one idle reading.
- Investigate the intake and crankcase-ventilation system for leaks using an appropriate procedure.
- If P0174 is also present, include both banks and shared components in the investigation.
What this code does not prove
The oxygen sensor supplies feedback used by fuel control. P0171 therefore does not prove that sensor is faulty. Bank 1 is also not universally the same left or right side of an engine.
Related fault codes
Sources and interpretation
This explanation was editorially cross-checked against the technical sources below. Manufacturer documents concern particular vehicle applications; their thresholds and repair procedures must not be applied universally.
- General Motors: P0171 / P0174 engineering information (2017)
- Delphi: Making sense of your sensors — MAF sensor
- Ford: 2013 MY OBD System Operation Summary for Gasoline Engines
OBD-II code data: OBDex (CC0 1.0) · 196fddffb6ef
Editorial source review: .
The causes listed are possibilities without a likelihood ranking. This page cannot replace diagnosis of the vehicle. A code alone confirms neither a failed component nor that continuing to drive is safe. The actual condition, warnings, handbook and applicable manufacturer diagnostics determine the next step.