Code P0420 catalytic converter

Not much to do other than verify that there are no issues like rich fuel conditions or other engine problem that could cause the sensors to read wrong. Use a scan tool to watch the pre and post catalytic converter O2 to see what they are seeing in the system. A five gas test at the tailpipe would also confirm if it is a bad converter (which 95% of the time it is).
The only real repair is to replace the converter(s) that are causing the code.

Good Morning my name is Roy and I will assist.

To add from Steve there is a way to confirm if you have a scan tool.

You need to monitor the downstream O2 sensor readings. If the readings are not steady then the catalytic converter is not processing the gases. If the catalytic converter is doing its job, the reading will be steady. It will not effect the running condition of the car.

Very common code for catalytic converter. It just means it is worn out and needs to be replaced.

Roy

How the converter fails determines its effect on the engine running. In your case the coating inside the unit is worn and it is no longer working. That could be from age (most likely) or possibly the engine is running slightly rich or burning a small amount of oil, both can cause the converter to fail. This failure mode will not really change how the engine runs it just sets the code.
Now, if the internals fail mechanically and get melted or broken, then it can cause a restriction that will change how the engine operates. In those cases you can have poor acceleration or large power losses, engine running hot and other issues. That is because the exhaust cannot leave the system and the engine chokes on it.

Before you replace anything be sure to hook up a scan tool and watch the O2 sensors before and after the converter. If this happened after a bath it could be that water entered a connector and is causing bad data. You might want to try disconnecting the sensors and spraying some contact cleaner in them. Nothing to lose and it might help.

Yes, unplug the O2 sensors and give them some contact cleaner and plug them in/out a couple times. Then see if there is a difference. If water got past the seals and is causing the sensors to give false readings you could get a P0420 when there is not really anything wrong.