if you are able to monitor your oxygen sensor, what you can do is monitor the voltage of your cat sensor when engine is in closed loop it should be around 800 mV and it should not change like your #1 O2 sensor from 200mV to 800mv. If your #2 O2 is not close to 800 and all the time this can tell you something about your cat. If it is brwn in collor that is a sign of overheating, also if you hit your CAT with rubber hammer from underneath and if you hear something like a sand inside it is bad.