temp sensor prob ?

After havaing my 2000 cavalier repaired when I blew a head gasket, my mechanic replaced the water pump, therastate, pressure checked and shaved the head and new gasket set the car ran great. The head wasn't cracked. I had to drive this car right away bck to buffalo ny from atlanta. 10 hrs into a 16 hr trip, I got the check engine light on and the engine temp gauge had been dropping down to the lowere temps of the scale.- around 110 or so. But the car ran great , and I was consistenly driving 70-80 mph all the way. After arriving home the light was still on but the car still ran terrific. I had talked to the mechanic a couple of times after and he is pretty sure the sensor may have been fryed and needed to to be replaced . Any suggestions? and were is this sensor and can I replace it my self? 2000 cavalier, 5 speed, basic model.

Which sensor are you talking about? What type of engine do you have? There are several on the Cav.

Which sensor are you talking about? What type of engine do you have? There are several on the Cav.
Sorry, I have talked about this problem with friends and assume everybody in the world knows about this. the engine is a 2.2 L ovrhead cam( I think on the ovhc) 2000 cavalier. Basic model, 5 speed standard, nothing power except steering. Their is only one gauge in the dash and it's , I assume , the engine temp or coolant temp . the ranges are 100-250 degrees. or so. Can you figure it out with this info.?

Have the codes pulled for the check engine light.