P0171 = System Too Lean Bank #1 (Cylinders 1-3).
Note, just because you only have this code, does not mean the entire engine is not running too lean.
Without being able to read fuel trims and getting more data, it is hard to tell if your have more of a issue that may be impacting the entire engine.
Assuming you really only have P0171, then you either have a vacuum leak impacting the front 3 cylinders, which is possible, however, unlikely. Or you may actually have a fuel pump issue. The fuel rail enters from the rear of the engine, so fuel delivery could possibly be an issue as well.
I would start with sorting the typical vacuum leaks then move on from there.
See the first 2 links below in my signature.