2004 350XG Check Engine Light - Evap. problem
 

I have had my car into the dealer twice in four months for the check engine
light coming on. First time they said it was an evap problem, they repaired
and cleared light.

Today, had it back for the same check engine light and was told it was an
evap. problem code came up as DTC PO456....however, this time they said they
could find nothing wrong. So, they cleared the light...

What is my next step??? Dealer said they find nothing wrong and if it
happens again they will have to call Hyundai for help.............Is there
just something they are overlooking?

Thanks for any information you can give.

P.S. Car has 28K on it and other than this has been great for me, no other
problems.