Eclipse engine temp and service engine soon

Hi I got a used 2000 eclipse GS two months back. Now the service engine soon light is on. I checked out the code at Autozone and then from a mechanic. Code says for bad catalytic convertor. Mechanic says it can due to catayltic convertor or rear oxygen sensor. This light is on for one month and i cant decide what shld I do.
Two days back the engine temp indicator started acting funny. started going up and down. But when I am on the highway the indicator remains where it shld be, there is moving up and down of the indicator.
Can it be the thermostat?

Please Help

ECLIPSE

if the service engine soon light means that you have a bad cat, the next thing you should have done was replaced it. try replacing the cat and see if that fixes your problems

well, either you got a bad cat and your thermostat is stuck, or. . . . . your cat is pretty much clogged and the extra backpressure that is building up heat in your engine causes the temperature to reach righer than it should.