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27/08/2022 7:29 pm
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Hi Scotty, our 2018 Ford Expedition has been one problem after another. But since about 95k miles (we have 120k on it now), we’ve battled the P0420 CEL. Shops have been no help. We replaced the cats with an aftermarket high flow set, replaced the O2 sensor on that side, used spark plug foulers in the after-cat sensor, changed coils, ran premium fuel and gas additives…still a CEL. When the CEL is tripped, performance and gas mileage take a hit. What next? Change the plugs again? Thanks!
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27/08/2022 9:36 pm
did you verify that the O2 sensors work before replacing all that stuff?
also try searching the forum. P0420 is covered in some depth in the FAQ. Also , the same question was posted a day or two ago:
https://carkiller.com/scottykilmer/qa/never-ending-p0420/