I have a saturn that both the cats went bad on and I didn't have the money at the time to replace them so I cleaned them both out and the car ran great no problems at all and it's an odb2. When it came time to smog I replaced the second cat only and it passed with flying colors and that's how it runs today. The only thing cats are for is smog. You will get a cel with empty cats but that's all mpgs and performance will be the same as if you had new cats on the car.