Getting a P0420 code. I know it means CC is below threshold, how do I tell if it's bad CC or o2 sensor. Also, could it be other things causing it?

 

420 dude.

Pull your spark plugs and check them, also check for any intake leaks. OBD2 scan for LTFT threshold and if it's above 15% you've almost certainly got a vacuum leak, smoke test the exhaust for leaks.

If everything looks good you might have a catalytic converter starting to **** the bed.