I'd suggest if you didn't do it yet to reset the system and see which codes reappear - just to be sure the set of codes you have represents what is really going on.

If the cat is not working to clean the unburned fuel this is detected in the OBDII system by a comparison of rear vs front oxgyen sensor signals - if the system's internal criterion is not met one or more codes will set. These may not tell you directly what is defective - they only mean the cat isn't doing its job. If you clear the codes for this they may reappear after some days as they are set based on a set of driving cycles.

Usually if one of the o2 sensors is bad a more specific code will be set - such as for slow response, heater circuit not operating, etc. Evidently from various posts on this board the sensors do get tired after some years of use.

The cat may not work right if the engine is running too rich or too lean - the fuel is then too dirty for the cat to cope with. If that's the case then fixing up vacuum lines, ignition, fuel pressure, MAF sensor, etc would be the way to go. These things do loosen up over the years. The cat may not be defective but will go bad if it is fed with very dirty exhaust for a long time and becomes saturated with crap. With vacuum leaks, etc., sometimes the code you get (if any) is also not exactly related to a particular part. For instance with a big vacuum leak, you could get a MAF sensor code because the sensor can't cope with the leak, even though the sensor is still ok.

Another possibility is that your cat or something else in your exhaust system is clogged up and restricting flow - this can happen as they age and fall apart internally. I had one car that did this and it would stop running right after a half hour or so on the highway once the cat got really hot.