jetrinka, I bought a used 2001 Focus SE back in December that was giving me that code and replacing both O2 sensors fixed the problem. I bought it knowing that code was set and assuming I would have to replace the cat, so it was a nice Christmans present from the auto gods i guess.
I know what the theory says but this is one area where practice differ in how Ford implemented it. In fact when I went to buy the sensor I had the OReilly parts manager confirm the code read with their scan tool (since I use a cheap $9.99 Chinese USB-to-laptop OBD scanner) and the parts guy said that usually only changing the front sensor was needed to fix that problem. I did both because I figured if one was bad the other was at the old rage stage also.