P2096 Post Catalyst Fuel Trim System Too lean Bank 1.

i posted a thread about this code during the summer. all the answers went to my rear o2 being bad. i replaced it and it went away till last monday. i erased it and then it came back sunday. im not sure if it is the heater element going bad with the new one or what. right now im just trying to think what everyone might think what it could be.