P0171: your engine is running lean
Running lean means the engine is getting more air, or less fuel, than the engine control unit is aiming for, and this code means bank 1 has been lean for long enough, and by enough, that the computer's own correction has run out of room. The computer constantly nudges the fuel mixture using fuel trim, and it sets this code once that nudge hits its limit without bringing the mixture back into range. It's a symptom the engine reports about itself, not a statement about which part is to blame.
You might notice
- The engine warning light comes on
- The engine may idle unevenly, hesitate, or feel like it's surging slightly
- lean
- — getting more air, or less fuel, than the computer is aiming for
- fuel trim
- — how much the computer is adjusting the fuel mixture away from its baseline setting
- bank 1
- — the side of the engine that has cylinder number 1
What the code itself says
P0171 is a powertrain (engine, gearbox and drive system) code, and the 0 after the letter means it is a generic code with a standard meaning.
Grouped here as: Engine running, fuelling and misfires — Air metering, fuelling, ignition, misfire, idle, boost, knock, oxygen sensors and the catalyst, all working together in the running engine. That grouping is ours, to help explain the code. The code's official family is the letter at the start.
This page is generic. Your car isn't.
A code on its own only tells you what the sensor saw, not what's actually wrong — and that changes next to your other codes. Read this one together with the rest of your scan and we'll work out what's really going on.
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