DML FAQ : Codes : P0455
Evap Leak Monitor Large Leak Detected
Leak has been detected in the evaporation system.
From Chuck Robbins:
Basically the evaporation system is a containment system for fuel fumes. The most common cause for this code (If memory serves me correct) is a faulty gas cap.
From Bob Smith:
If everything else checks out, and you still get this code with the gas cap tight, check the O-ring (seal) on the cap. It may have dryrotted or otherwise be damaged
From Bob Tom:
I would check the electrical connection and the condition of the connector at the evap. purge solenoid (driver's side bolted to the firewall) just to make sure that solenoid can function as it should (won't help if solenoid is plugged or bad though). The PCM controls the ground side of this solenoid (it is normally open). When energized by the PCM, the vent solenoid will close, sealing off the fresh air side of the system. The PCM only does this when it is running its evap monitor (self-test).
See http://www.motorage.com/edindex/109828.htm for a far better description of the system.