Used an O2 spacer. P0420

OEM replacement converter? Federal car or Cali? Any exhaust leaks? Do you have a scan tool that can read the live data from the sensors? All stock, no cold air intake or other modifications?

Was the vehicle doing anything out of the ordinary when you got this code previously, before you replaced parts?

Have you checked for exhaust leaks?

Also an easy way to test the system, is to pull the upstream o2 sensor out, install a back pressure tester, start the engine and see what the back pressure tester reads? This is a quick way to tell us if you have a bad catalytic converter or not and how to proceed.