I've got a 98 four door two wheel drive blazer with a 4 speed (460le) and for the past 6 months my exhaust has been making a deep rattling noise that i just attributed to a loose or missing bracket somewhere in the piping. however within the last 4 months my fuel economy has dropped from about 20 mpg to an average of 14 and my engine's performance took a moderate dive. Soon after my check engine light popped up periodically and it gave me the error code P0420, which is a catalytic inefficiency.

promptly after i gave the truck a well needed tune up, i changed the plugs and wires and the fuel filter. upon inspecting the old plugs it looked like the mixture was running too rich based on the deposits. i know that the truck does not have a vacuum leak, and the exhaust manifold appears to be in good condition. still the poor gas mileage and bad acceleration remains. with those factors ruled out the only obvious causes would be faulty O2 sensors or a bad cat.

how would i go about diagnosing this problem or would there be a possible cause that i have not even considered (pcv valve, air intake, fuel injector, flux capacitor?) and keep in mind that as far as auto mechanics goes i know how to turn a wrench but i'm far from an ASE certification, so take my observations with a tiny grain of salt.

any advice would be appreciated, thanks.

 

Don't forget your halogen fluid for your headlights!

An exhaust back pressure test could be done to determine how restrictive the converter has become, but all of your symptoms point directly at a failed catalytic converter.