04 mazda 6 3.0 Cat
 

I have a 04 mazda 6 with the 3.0. It seems to have lost power I am thinking it's the Cat's. I read a old post here but my question is how to tell if its the cat or pre cats that are bad? or do the pre cat's not usually go bad? I am going to dig into this this weekend. I am just looking for advice.