A loose gas cap most certainly can cause this code. Follow this link:

http://www.obd-codes.com/trouble_codes/generic/p0455-evaporative-emission-control-system-leak-detected-large.php

Yes, the filler neck can be replaced if necessary (assumig that Ford has not discontinued the part.

I'm also experiencing a "tight" or hard to screw in gas cqp so please let us know if a new cap solves the problem or if you need the filler neck too.

Jim Johnson 98 SVT 03 Escape Limited