P0171 is a lean code an could be caused by several different things besides a vacuum leak , Bad O2 sensor , mass air flow sensor , stuck PCV valve etc...
http://www.obd-codes.com/p0171
P0172 is a rich code , here again could be several thing's - mass air flow sensor , injectors , O2 sensor , fuel pressure regulator etc...
http://www.obd-codes.com/p0172