My wife’s car started running a littler rough over the weekend she has a 2001 Subaru Forester it’s unmodified, and has had a CEl for P0420 witch is a general code for catalyst below threshold, it was this way long ago when we got the car and it ran fine then. Now under acceleration it hesitates and is a bit sluggish getting going. However it did not give any new codes ( I was expecting to see a misfire code) so my underlining question here is how do you tell if the cat is bad or if the o2 sensor needs to be replaced?
Your first mistake was asking Melissa... you need to "take it to Al" tomorrow, have her take the xB and then cut it out and test pipe it. Also, good chance you can remove the O2 sensor, hit it with a torch, wipe if off and it might be good.
It's a possibility that the cat has fallen apart and is blocking airflow. A new cat is big $$$. Cut it out!
With only a P0420, I'd be looking at other things. Plugged cats are not common, far more likely a general tune-up thing. Consider age and condition of plugs, wires, fuel filter, etc. Watch live data to verify coolant temperature sensor, and fuel trims.
Drive the car today to do an oil change for the wife and the CEL came back on with P0303 cyl3 misfire, plug looks gross we did replace the ignition coil pack maybe two years ago , but did not change the wires or plugs
