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CODE P0420 on 2005 Ford Expedition

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
HAVE CODE PO420 WITH CHECK ENGINE LIGHT REPLACED RIGHT BANK CAT AND HEATED OXYGEN SENSOR BEFORE CAT. WHAT ELSE COULD BE WRONG?
(2) Answers
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.
We have same model, same problem. Ever figure it out?