Failed the DEQ test with a trouble code P0421. Had a local mechanic do a cat test, he said the cat was clogged but using premium fuel whould clear it up and to try DEQ again. Failed DEQ again with the same code and then got a cat test by a franchise mechanic, they said I had to replace the cat and oxygen sensors. The fanchise place had a pretty high service charge so I priced another local mechanic and he said that my vehical shows two cats, one in the intake manifold and one in the exhaughst pipe and he reccomends doing a diagnostic test to see if it is one of the cats or just the oxygen sensors. Now what?