Gremlin P0171 code, won't go away.

I’m getting a P0171 code, “System Too Lean (Bank 1).” It’s the only code I’ve got, but it keeps activating the service engine soon light. I’ve replaced both O2 sensors, cleaned the MAF, and fixed a vacuum leak in the PCV system. Code still won’t go away, and my fuel mileage sucks (approx 27 mpg, mostly freeway).

Any thoughts on what it could be? Maybe I still have another vacuum leak somewhere. The car runs pretty well, but the computer is probably running it really lean, so it’s probably a little down on power, and probably the reason my fuel mileage sucks. Thanks in advance.