Does it have a rattleing sound? or a smell of sulfer of a burning from time to time?
I have an 03 Focus svt and i had to get the catalytic converter replaced. Luckly it has its own 8 year - 80000 mile warrenty.
Did you take it do a Ford dealer for the light check?
OHHHH, when i first got mine...i had the same problem...they had my car for like a week and couldn't figure it out. I went to 2 different dealers. The second figured out that the wire/s coming from the O2 sensors had fallen on to the engine and were being shorted out!!!
Have them check that!