Eclipse engine temp and service engine soon

[Eclipse 00DB]

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

[eclipsed4utoo]

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

[perna00]

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.