Check engine light, code P0455

about a month ago, i got a gas cap light. it went away after driving a bit. 200 miles later, i get a check engine light. i got one of those scan tools for the engine and it says code P0455 "evapoative emission system leak detected. (gross leak / no flow ). i dont know what part needs replacing or if i need to take it to a shop? i’m going to try to get a new gas cap but i need other suggestions.

forgot to give my make and model. 2008 chrysler aspen 5.7L v8.

The code can also be caused by a bad vent control valve if the gas cap replacement doesn’t work.

Tester