See here:
http://autorepair.about.com/cs/troubleshooting/l/bl_obd_main.htm

Vapor leak in the evap system.  It keeps the gas fumes from escaping
into the atmosphere, very important in some states for inspection to
pass.  From what I've read on the subject... on Ford vehicles engine
vacuum is used to detect leaks in the evap plumbing.  The PCM directs
the vacuum solenoid to pull a certain amount of vacuum and it must be
held for a predetermined amount of time, or else fails.  Two failures
sets the code and engine light on.  Gross leak is not good.

Check your gas cap is on tight.  Otherwise, check the evap lines for
cracks, damage, etc.  Evap is very tough to diagnose properly on your
own, could be any one of a number of components in the system,
including the gas tank, fuel filter seal, canister purge valve,
charcoal canister, lines, etc.  Might be worth a trip to the dealer or
shop.

In article
<1159237197.811714.292890@m73g2000cwd.googlegroups.com>,

The evap canister on a 98 Escort is in the left rear wheel well
behind the tire, the canister assembly has a vent valve, check it
for bugs and other debris that would keep it from fully closing.

Most of the P0455s I see are because of this, YMMV.

Good luck, the canister and attached valve is no longer sold by
Ford.