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Re: GM dealer says P0420 caused by downstream sensor. What say you?
Posted to Technical Discussion Forum on 3/10/2007

Something like... if the O2 is still the original one and now with the cat new, if the code never returns, YOU WERE RIGHT. If the code returns TOU WERE WRONG.

I would be interested to find out one year from now where this has gone.

Andy Login to read more.