Q: Could a manifold gasket leak make the catalyst system read PO420? on 2000 Subaru Outback

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I have a manifold gasket that keeps blowing I think because the block is aluminum. Since this lets out emissions before getting to the catalytic converter. Could this be the reason I am getting a low threshold reading? When I turn the engine light off it takes about two weeks before it comes back on and usually when highway driving.
(1) Answer
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P0420 indicates the catalytic converter is performing below a predetermined threshold. The performance is measured by the rear oxygen sensor. If the exhaust manifold is allowing air to enter the exhaust system, that excess air can mess with the measurements made by the oxygen sensor. So to answer your question directly, yes, it is possible than a manifold leak could cause a false code P0420. However, it is not likely. There is also an oxygen sensor in front of the converter. If there was excess air in the exhaust, I would expect the front 02 sensor to see the extra oxygen and set a code for exhaust too lean. If that is not happening, then excess oxygen is probably not present and therefore, not responsible for your code P0420.
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