My 08 has 2 cats on it, IMO i would say its the coil. Mist water over it and that will tell you. If the coil is bad, that would cause the cat to go bad. ( happened to an 02 ford tarus, coil went bad and took all 3 cats with it. ) Why fix one thing that is an issue and come back to the original issue that caused the issue in the first place? To sum it up, check your coil... Mist it with water. Just stand back you will not get shocked...