Please hep with check engine light and code P0171 system to lean

[keng]

Hey guys I am having trouble with my 1998 explorer 5-speed. My check engine light came on and I ran a dianostic and it came back with a code P0171 or Po171 which are system o lean. What could be causing this?
What should I check.

 

[Blee1099]

Could be a number of things, dirty MAF, bad O2 sensors, etc. Try cleaning the MAF sensor with some electrical contact cleaner and see if the codes go away. Also what motor? 4.0 OHV or SOHC?