For what it's worth, when my water pump seal blew and my engine overheated
one of the several expensive side effects was that my cat melted.  There was
lots of rattling little pieces inside and the exhaust also got plugged up.
Check to see that your coolant is properly circulating (maybe thermostat is
stuck etc.)

Mark

>
>On the bad catalytic converters.  The failure mode is the inside bonding
>material breaks down and the converter ends up in little pieces and some
big
>pieces that plug the exhaust to the muffler.  The first time this happened
>was when my wife was on her way from California (where I live) to Oklahoma
>for a dog show.  The cat converter was replaced in New Mexico.  Lasted less
>than a year and I had to pull the muffler to unplug it.  It still has small
>pieces in it and I need to get into it again and clean it out