420 is catalyst system efficiency below threshold. Just went through that one. Went the cats start to go bad it will trigger that code especially when driving slow where the cats don't stay lit up. Eventually it will just trip all the time.

There is one cure and only one. Replace the cats. If you are in CA there are two cats. Non CA models there is one but CA models are sold outside CA. Don't bother visiting the dealer. They are at least a $1000 each. I shopped around and the best I could do was Midas for about $1000 for both installed. Outside CA you can buy them for $200 each and put them in yourself. They just bolt up.