The best thing to do before randomly replacing items you may not need to is to do some diagnostics. Check your cat with a simple gas ranging meter (like a carbon monoxide/dioxide meter but not quite for those) and see if they jump through the roof before it hits your CAT. Also, an O2 sensor is supposed to give off 0v-5.0v when it is working properly. Test it with a DVOM. Simple stuff. If it's in range it's fine.