if you ever had the inside of a cat apart, its a very simple thing. it is a grid of tiny squares the length of the cat. making the exhaust flow through it. this grid is a ceramic material. when you run rich, it puts unburnt fuel in to the cat. when this happens, it burns in the cat. casuing it the melt down and plugging it. also repeated heat cycles can make it brittle and falling apart inside. this will cause a rattle and can restrick the exhaust causing lose of power. if you run with a restricted cat for too long, you can burn valves, or even catch the car on fire due to the excess heat in the cat. a cat has one purpose, with the heat in the cat, the material that the cat is made of, it creats a chemical reaction iside of it. this is why you see water coming out of your tail pipe.
exhaust starts out as carbon monoxide(CO )and hydrocarbons (unburnt fuel) after going through the cat. it converts it to carbon dioxide (CO2) and H2O(water) both of which are nontoxic.
fuel managment is very important when it comes to making you cat survive. anytime the SES light comes on, you need to have it fixed ASAP . the pcm tries to keep your air/fuel ratio at 14.7:1 this isnt for performance bu for the EPA to be happy.
hope this helped understand how a cat works and how to make one live