P0171 fault code: fuel mixture too lean (Bank 1)
Plain-language definition
System Too Lean Bank 1
What does it mean?
For the engine to run cleanly and efficiently, air and fuel have to mix in a specific ratio. The engine computer constantly corrects the amount of fuel it injects based on readings from the oxygen (lambda) sensor in the exhaust; this is called fuel trim. If the mixture is lean, meaning there's too little fuel for the amount of air, the engine computer adds fuel.
P0171 means this correction has reached its limit on Bank 1: the engine computer has to add much more fuel than expected to balance the mixture. Bank 1 is the side of the engine with cylinder 1. Inline engines usually have only one bank, so on them the code covers the whole engine.
Often there's no obvious difference while driving. You may notice rough idling, hesitation when you press the accelerator or a loss of power; if the mixture gets very lean, misfire codes may appear alongside it.
How serious is it?
This is a Warning-level code. If the cause isn't fixed, the mixture can get even leaner, which can lead to misfires and the engine running unevenly.
Severity is a guide; if the warning light is flashing or you notice symptoms while driving, have the car checked at a workshop.
Common general causes
- Air leak on the intake side (vacuum hose, manifold gasket)
- Dirty or inaccurate air flow sensor (MAF)
- Low fuel pressure (fuel pump, fuel filter)
- Clogged or dirty injectors
- Exhaust leak before the oxygen sensor
- Oxygen (lambda) sensor giving wrong readings
The list shows general causes; a workshop finds the actual cause on your car with a check.
Definitions by make
Some manufacturers record the same code under their own wording:
- BMW
- System Too Lean
Related codes
How does it appear in the 2dakika report?
System Too Lean Bank 1
If the code is read as a confirmed (stored) fault, the report shows it with the "Warning" severity; in pending or past records, codes that don't request the warning light and aren't critical appear as "Info". If the engine computer stored them, the conditions when the fault was recorded (engine temperature, rpm, speed) are shown too.
Read this code with the OBD-X1, and clear it from the app after the repair.
Clearing the code doesn't fix the fault; if the problem persists, the code comes back.