I have a 2018 f350 and I connected a. Device to my truck and this code can up PO420 and i don't know what that means if any body and help me llfeel free to help me thank you.

 

There is a problem with either the catalytic converter or O2 sensor after the converter on the passenger side piping.There is usually a small converter and a larger converter.
The code suggests the problem involves that second converter area.
Converters don't usually decide to go bad, there is usually an underlying reason `and a good shop should be able to pinpoint what caused it all.

If the converter needs to be replaced, they can cost a bit more than what we might expect.