engine light is staying on on 2004 Kia Optima

I have the carmd and it is telling me the reason for light is gas cap. I replace cap and had it reset at a local kia dealer. Now 2 weeks later light comes back on with it stating the same problem. Anybody out there having the same problem. kia states it is also the gas cap. inspected gas cap and it is in perfect condition

Asked by for the 2004 Kia Optima
Kia as a car manufacturer put a lot of great technical information and work shop manuals on line for free for Kia owners. I'm sure it some sort of an evaporative emission control problem but what is the exact code they found.
P0455 is the code that came up. thanks for replying