Got a Carrier rooftop, model 48TM008, that is going off on the limit switch. Another guy changed it in December, and today was the first time, that I know of, that it went off again. I put my digital thermometer in the box so it was as close to the disc as possible. The unit ran for the hour and a half I was watching it. Temp on the digital got up to about a max of 172F. It's rated for 130, with a 40 degree diff. Whats the normal operating temp for the limit? Seems to run fine during the day, but when it does go off, it is either late at night/early morning, because we get the calls first thing in the morning. Could we possibly be looking at a bad heat exchanger, leaking and heating up the disc? Any thoughts? Thanks.