It will come back on, at some point...normally does..

Usually Fords get this code if they have had a misfire due to failed coil pack or whatever,with excess unburnt fuel going into the cat.. then after the misfire is fixed the next thing that can happen is the EML light comes on with the P0420.

Ideally needs the pre & post cat lambda sensor live data checking in graph form to see if the pre & post lambda signals are the same. If the cat is fooked, both the signals will be the same, if the cat is good, you'll just get a constant voltage on the post cat lambda while the pre-cat lambda is switching once in closed loop operation.