2001 Ford Windstar Emmission Test Failure Codes P0171, P017

I just failed my emission test and have purchased a computer to read the codes and to reset them. I had them reset, but I am not "Ready" some emission tests have not completed their cycle. I was trying to determine what P0171 "System too Lean Bank 1" and P0174 "System too Lean Bank 2" would mean terms of a repair to the vehicle. Any help would be great!