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P0423 CAT inefficiency

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Hey All

So a little bit of context, I have a Yaris 1.3 2006 with 150K miles and I am getting a P0423

Over the last 6 months I’ve had the CEL light and thought rather than change the CAT, replace both o2 sensors as a process of elimination. Interestingly, when changing the rear sensor it seemed to have temporarily fixed the issue for a month or so but it came back again. So now I’m thinking it’s the CAT. However, there are two in the exhaust system. One on the header and on just before the centre box. So the question is, is it the manifold with the built in CAT or is it the downstream CAT? Or possibly both.

Please reply with any experiences or solutions.




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