You dont say what year your fiesta is, but by the symptoms its probibly '02-'04?
EAC stands for electronic accelerator control-related to the accelerator pedal and fly-by-wire accelerations system.
Cures varied from a simple ECU update to a ECU replacement, depending what combination of fault codes you have stored.
I would take it back to the dealer and ask them to note for you which codes you have stored-you WILL have some.
Once you have them it should be easy to determine what course of action is required.
Hope this helps.