p0457

Hello everyone, i have a 02 mustang 3.8 v6 where the (check gas cap light) is on and had a code of p0457. I have not replaced the cap yet, the o-ring looks good in it and i have read where others have had to replace the rubber gromet where the fill neck goes into the tank. Is there any common things to look for on the mustang before i start tearing into this thing, i have also noticed some gas smell when the windows are rolled down and the gas milage seems to have gotten terrible. thanks for any help you guys can give me