Code P0420

An O2 sensor is a lot easier and cheaper to check or replace than buying a new catalytic converter. You may also want to look for any exhaust leaks, leaking injectors, or timing, as those can also trigger a PO420 code. You can Google "how to test O2 sensor" and "how to test catalytic converter", but just so you know getting an O2 sensor out of the exhaust can be a challenge for a home based DIYer. There's a video on Youtube as well on checking catalytic converters for blockage using a shop vac. Most likely your problem is going to be with the sensor or one of the other issues listed above and not with the cat.