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Hi, I have a 2004 Honda Civic dx.... and my exhaust mainfold was turning red when i reved it, so my father-in-law took a screwdriver and started scrapping all this grey clay powdery stuff out, he was saying that it was clogging it, but then i was told it needs to be in there, so my question is, will all the stuff he took out affect my gas mileage?

 
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Sounds to me like he gutted your cat. It was probably clogging it at the time, but your car's gonna throw a CEL now, probably a P0420 (catalyst system under efficiency threshold)

 
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yea thats what exactly what happened... i saw the scanner say that when i took it to a mechanic, so by him gutting it wont affect my gas mileage right??

 
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Don't think so.. Some people gut their cat so it increases airflow through the exhaust system. You just won't be able to pass emissions unless you either get a new cat or you do some O2 simulator stuff..

 
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oh ok....thnk u very much, u put my concerns 2 rest.....lol