Drive with caution — fix soon.
P0171 means the engine is running too lean on bank 1, meaning the air-fuel mixture has too much air or not enough fuel. The computer noticed it's adding extra fuel to compensate but still can't reach the right balance. On most engines bank 1 is the side that contains cylinder 1.
$100 – $550
Varies by vehicle and root cause.
Usually yes, for the short term, but don't let it linger. A lean condition can lead to misfires and added heat, so have it checked soon.
It varies with the cause: cleaning a MAF sensor or fixing a vacuum leak can be $100-$250, while a fuel pump or injector job can run $300-$550 or more.
It's moderate. It won't usually strand you right away, but a long-term lean mixture can cause misfires and engine wear, so it shouldn't be ignored.
Unmetered air getting into the engine is the most frequent cause, typically from a vacuum leak or a dirty mass airflow sensor.