New 2012 Elantra touring, less than 600 miles.

New 2012 Elantra touring, less than 600 miles, Check eng. Lgt. Came on, code:P0449, evap system. Dealer said loose gas cap, Cap tightened, code cleared. Two start ups later, light back on. Brought back to dealer they checked the evap system, no leaks, changed stuck valve, cleared code. Next day light back on. Now they are telling me that sometimes the code doesn't always clear and it sometimes stays in the system and that's why the light goes back on. They told me to bring it in again and they will clear it agian. This sounds like crap to me. I thought that when a check eng. Light goes on, the computor has sensed something wrong. When they claered it and the light went out that means the system is cleared. Am I right and what should I do next?
Thank you!