2002 Saturn L300 thermostat diagnosis

My service engine light came on a few days back and I was told one of the reasons was the thermostat malfunctioning. I've never had any problems with the car overheating and I thought a faulty thermostat would certainly cause the car to overheat. I thought there would be some more obvious signs of a bad thermostat and the car was running fine. I was quoted somewhere in the neighborhood of $900.00 to fix this so I decided to research it first. I was also told that if I didn't replace it soon and continued to drive the car the catalytic converter would be damaged. Is there any truth to this? Thank you in advance for taking the time to respond.