when the cat went in my cougar i could hear the broken honeycomb pieces rattling around in the housing when i would rev the engine. also, the exhaust got louder and sprung a leak right next to the converter because it couldn't get through. this is an extreme example. i ended up gutting the precats and replacing the main with a high flow unit. what a difference in power that made. if your car pulls out with less umph then it used to and you experience any other symptoms like these then its your cat. they usually have a life of around 80k anyhow so even if it's not the cat i'm sure it needs to get replaced anyway.
