Engine light stays on . I have a code P0455 what does it mean .

The most common things to cause P0455 are the fuel cap,evap hoses, purge solenoid, and vent solenoid in that order. I'd start by replacing fuel cap from the dealer,which you've already done and if that doesn't help use a vacuum tester to see if purge solenoid and vent solenoid both hold vacuum and replace whichever one doesn't. you need to get it on a lift if possible...if you can't do this work yourself,take it to a shop and have it smoked.

P0455- Evaporative Emission Control System Leak Detected (large leak)...With this code being given, the best way in order to determine what is wrong is buying doing a smoke test of the system. By smoke testing the system we would be able to determine if there is a solenoid not operating correctly or if we have a vacuum leak somewhere in the system. Please give us a call at 847 352 2886 (ask for Scott or Mike), so we can set up an appointment to look at the vehicle and determine what the root of the problem is!...Thank you and we look forward to hearing from you!