troubleshooting code p0455

I just got back my '98 dodge grand caravan from the garage.They replaced the egr valve and solenoid on it.There is a brand new gascap and 2 new o2 sensors.The check engine light came on again.Took it to autozone to get code read.Came up with p0455.This van has been in the shop at least 4-6 times for the check engine light.The warranty runs out on 6/20/08. Why didn't the mech find out this code?It has a different motor put in it in march of this year.Can any explain this code or a fix for it.:confused:

It means the Evaporative Emissions Control system (EVAP) has detected a major leak. A loose gas cap is the usual cause but, another could be the mechanics may have accidently disconnected an EVAP system vacuum line. Some EVAP system tubing and EGR system components are located in the same area. Any idea what codes were turning on the service engine light the other half dozen times?

http://www.obd-codes.com/p0455