[thephantom1492]

I agree with GTP Dad. A cat ain't supposed to burn out. The 2 usual cause are too much fuel and burning coolant.

The goal of the cat is to burn the unburn/partially burn fuel. Doing so it become hotter. They size up the cat based on the expected fuel to burn. If your O2 sensor is wrong it can let too much fuel in the engine, making it unable to completly burn it, causing the cat to overheat

As for the coolant, I don't have the full explanation, however it seems to be a chemical incompatibility between the coolant and the cat, causing it to get ruined. (but don'T quote me on that, I never did the full research, so it might be something else, but that is what I read somewhere)

If you need to add coolant from time to time then you have a blew up gasket or cracked head or something. If you get bad MPG, well, you would have your answer.