Engine Code Keeps Returning Code P0171?

I have a Ford Focus 2.0L Dohc and a code P0171 Keeps returning and the mil illuminates. I have replaced the intake gasket, idle air control valve(was having high idle) but that issue is fixed, i changed the m air flow, there was a pcv valve hose that was collapsed about a few months ago and thats fixed with a new pcv valve from the dealer, there dont seem to be no vac leaks, and i checked the egr valve what could be making this code returning?