Originally Posted by jcp123
Maybe. If the honeycomb melted and is clogging the pathway, it sure could impact your mileage. If all it is, is a substrate material which has been "used up", it should have only a minimal impact on MPG. Downstream O2 sensors barely factor into your fuel trims. If you only got p0420 today, I doubt the computer is correcting anything enough to affect MPG.
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