I recently purchased a used '99 with 76,000 miles on it. After driving it for 4 or 5 days the "Service Engine" light came on. I had it checked at Autozone today and was told that it is coding P0455. I checked that the gas cap was on correctly. Is there anything else that I can do before I take it to the dealer? The engine is a 5.4L.

You can try lubricating the seal on the cap with a little vaseline. That will help some if it is the seal leaking.

You will have to take it to someone that has a smoke machine.There are smoke machines to find EVAP leaks and the smoke machine does work finding EVAP leaks