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Hi I've been getting the code P0421 and low gas mileage. First thing I checked was if my spark plugs were fouled up. When I checked them, all my spark plugs had oil. So I replaced the valve cover gasket and spark plug well gasket thinking that might fix the problem. I erased the code and drove the car for 100 miles and no code popped out but, when I checked my spark plug wells again there is moist building up again and a little bit of oil. It also kinda smells like gas too. By the way, I tightened up all the bolts with the torque specs assigned by the manufacturer. Any ideas why? Hopefully not a bad head gasket or valve cover.
 

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P0421 is a catalyst code and has nothing to do with the spark plugs. It could be a fouled O² sensor, but chances are one or more of your cats is bad. Unfortunately, replacing one of those is not easy and may be pricey.
 

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Costly once manifold is done usually cats need replacing too, cost me $800 for new manifold and exhaust work when my cats were clogged . Next time im gutting everything lol
 
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