I've got a 00 Elantra that I'm trying to keep alive as a A 2 B car. The temperature started going up like crazy last week so on a guess I switched out the thermostat - no go. Has anyone ever replaced one of these? The replacement was a bit smaller than the original but it still fit, should I be concerned? I was also under the car changing the oil and the catalytic converter was very hot (the car had been off for over an hour) if it (the converter) goes could this cause over heating? Any suggestions?

 

well, you got the wrong thermostat. Don't stinge on the money and fit one from an HD-dealer.
Cat's get normally very hot, ant take some time to cool down. A defective converter generally gets blocked, and will block the exhaust flow = no or low engine power.