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GM dealer says P0420 caused by downstream sensor. What say you?
Posted to Technical Discussion Forum on 3/9/2007 30 Replies

Is there a possible scenario for a downstream oxygen sensor to cause a false P0420?

This is my Mother-in-laws car, a 2003 Pontiac Vibe (Toyota Matrix). I brought it into the shop for some maintenance and to check out her MIL. It had a P0420. Upstream sensor ... Login to read more.

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