[metroschultz]

Originally Posted by hummingbird

maybe a bit OT,
(1)but isn't a catalyst supposed to remain unchanged while helping chemical reactions?
(2)So technically the cat should not 'change' over time except for soot deposits if any, if at all... So what would 'age' the cat to require a replacement?
(3)Can't it be cleaned up and re-assembled in place, like could be done once upon a time?

Lots of other factors in these questions.
(1) Yes. However you are putting simple logic on a complex issue.
...The cat remains unchanged as long as;
Your engine is in tune and you don't accidentally add toxic chemicals in the fuel. (!!Lead!!, some cleaners in high dosages, etc.) This will lead to premature failure and an inability to do it's job correctly.
(2) Cats are made with precious metal bonded to a ceramic honeycomb structure. As they get older the metal will degrade and go away. I have seen cats with 200k on them that still worked well though, so all is dependent on the way your car reacts and you drive.
They do not like excessive vibration and really hate it when you drag them into/over speed bumps.
(3) I don't know of any cats you could open and repair. I know of the early GM units you could open and empty. ?!?!?!?!

So to sum up, your cat will last for a good long time provided you keep the engine running well and you don't abuse the cat.

Hope this helps,
S.