CODE P0420 on 2005 Ford Expedition

HAVE CODE PO420 WITH CHECK ENGINE LIGHT REPLACED RIGHT BANK CAT AND HEATED OXYGEN SENSOR BEFORE CAT. WHAT ELSE COULD BE WRONG?

Asked by for the 2005 Ford Expedition
2 answers
We have same model, same problem. Ever figure it out?
Did you change the O2 sensor before or after the Cat on your 2005 Ford Expedition? I know of one case (It was a Cadillac) where the rear O2 sensor, the one after the Cat failed in such a way to case a P0420 to set.