It is the after cat Lamda sensor at fault most likely switching too slowly.
A lot of modern cars have a LAMDA before and after the cat to check it's effeciency.
I would change the after cat one, then if successful get ford to reimburse you for being idiots.
There is a very good section in CAR MECHANICS, for problems of this nature, that you can email/write to as well.
Hope this helps,
tabetha