In reply to californiamilleghia :
For post OBDII cars, there's an O2 sensor downstream of some or all of the catalyst system, and that signal is compared to the main O2 sensor to see if the catalyst is damaged. FWIW, the threshold for lighting the light is 1.5x the standard that it's certified to. So when the light is turned on, the catalyst isn't completely dead, it's just not working well enough to mean the emissions are not meeting a specific performance requirement.
Many cheap catalyst don't have enough material on them to even meet that performance requirement, so the signal comparison is able to pick that up, and trigger a fault.
"Clogging up" of catalysts isn't nearly as common as people thing- that's generally a thermal even that is enough to melt the ceramic substrate. And that's normally caused by misfires- so you get a whole cylinder's a/f mixture, which is a lot of energy for a catalyst to take- do it enough times, and the energy is enough to melt the catalyst. That's when you get the potato in the tailpipe.
As I see it, most catalyst failures are either the metal sintering together so much that there's not enough precious metal surface area to maintain the chemical performance OR it's poisonings. And the poisoning typically comes from phosphorus in the oil anymore- lead poisoning is really rare.
