P0171 Code-- can someone help?

I am receiving the code P0171 (fuel system for bank 1 is too lean)
--any suggestions as to what this means or what fix is required?? Is it simply the o2 sensor??

I have already replaced the downstream o2 sensor. I am not the most experienced with this sort of thing-- I can replace a sensor or two-- but that's about it...


Any help would be appreciated!!
01 Jetta GLS; 60,000 miles (2.0)

check for vacuum leaks (fuel pressure regulator vacuum hose on belt side of engine under engine cover)

if there are no leaks replace the maf sensor