ford focus?

If you have had mis fire problems in the past you may have a damaged cat or the O2 sensor after the cat may be at fault also, it really needs to go onto a correct code reader to look at live data regarding the sensors in your exhaust,I would imagine a new cat would be quite expensive and before even thinking of renewing it would get this test done.You could also have a look at the wiring on the sensor also for breaks/chaffing etc

cdc
June 2013