Okay, I don't know what a 420 is. I can, however, tell you to ignore the Service Engine Soon light. Mine has been on for 2 years, it just signals a problem with your emissions system, not necessarily your catalytic converter. In my case, the light is on (although it switches off every once in a while) because I brought my car from the US to a country where we use leaded gas so I had the catalytic converter removed before I left. My husband, on the other hand, brought his brand new Dodge Dakota P/U and left the catalytic converter in, uses leaded gas as well (that's all they have here) and his car runs beautifully.
Whenever your next vehicle inspection is scheduled they'll probably say something about it, but you can drive comfortably until that time comes and that will give you the chance to sure up some money. Conversely, you figure out which of the little switches in your car's electric system is responsible for the Service Engine Soon light and take it out. The light won't come back on, and unless they actually check your emissions they won't have a clue during the inspection because nothign will show up on your instrument panel.