If you suspect your catalytic converter is bad, or you have a P0420, there is an easy test to check your catalytic converter’s operation. You first need to get your hands on an infrared thermometer or a thermocouple. Basically, you need something that can accurately measure temperatures up to 1,200º F.
When a catalytic converter is working properly, it will be 100º F hotter at the outlet than it is at the inlet. First, make sure the engine is up to operating temperature. Then run the engine at about 2,000 to 2,500 RPM. Take the temperature readings at the front and rear of the catalytic converter. You should see an outlet temp that's 100º F hotter than the inlet temp. If you don’t, the catalytic converter is bad and should be replaced.
I've heard that you can sometimes repair a bad converter, but I have yet to try any of those methods. Jury's still out on that one. I mention this because catalytic converters are often quite expensive. That's because they contain platinum, and platinum in their substrate. These are precious metals and are expensive. The expense of a new converter often holds people back from repairing the issue. If you live in a place where there is emissions testing, you will have to deal with it at some point.
I'm going to once again touch on aftermarket parts here. There are some universal catalytic converters out there that you can use as a replacement. Know that sometimes these won't work and will still set a P0420 right out of the box. So if you're faced with having to replace your catalytic converter, be sure to use a quality part for best results, OE (Original Equipment) if you can afford it. Here's a video demonstrating the converter test.