Help! Codes 1684, and 0455

I just bought my Intrepid three months ago from a used dealership. This morning, my check engine light came on. I did the key thing 3x, and received codes 1684 and 0455......I know 1684 isthe battery has been disconnected within the last 50 starts........I hadn't done anything with the battery........code 0455 is a large leak in the evaporative system.....someone told me about the gas cap...not that.......what do I do??