Check engine light...O2 sensor....Misfire?

New here and just wanted to ask if anyone would know what the problem would be. I have a 2000 Focus ZX3. First (autozone) diagnosed it to be the oxygen sensor. I replaced it and reset the computer and it went off for a week. It recently came back on and it is the same problem. They said it could be the catalytic converter, a misfire, or the O2 sensor(which it isn't cause I replaced it myself). The car runs great and has very good power. I had just replaced the fuel filter also the day it came back on....The spark plugs too. Anyone care to take a punch at it?

What are the code numbers

on these 4 cly. ford engines we have been finding cracks on the coil packs. replacement has cured the mis fire each time. also looking into the top of the coil pack i have noticed a discolored brass tab on the mis firing clyinder too....3 were shiny one was very dull....

I had the same problem last year with 03 Focus. It registered as an O2 sensor but when I took it to the mechanic shop they said it was a leak in the PCV vaulve. If you can replace it yourself I suggest it since a little 4 inch rubber molded hose cost me almost 300 dollars!!! Haven't had an issue since. but they did say the rubber does corrode and will likely need to be replaced again.