What is the code P 0171 in a Subaru Forester?

The code P0171 in a Subaru Forester indicates that the engine control module (ECM) has detected a lean air-fuel mixture in the engine, meaning there is too much air and not enough fuel in the combustion chamber. This can be caused by issues such as a vacuum leak, a malfunctioning mass air flow sensor, or fuel delivery problems. It's important to diagnose the underlying cause promptly to prevent engine performance issues and potential damage. A mechanic can use diagnostic tools to pinpoint the exact source of the problem.