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Q: 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?
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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.
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