Check Engine Code PO455

Hello Men,

I have a 1997 Taurus GL & the Check engine light is on again.
Came on last year "evap." so I replaced fuel cap -w- a new one and was good for about a year.

This time it's code PO455

Autozone said it's "evap. leak major" Any Ideas where I should start?

Thanks in Advance, to you all.