There are several thermal sensors in every mac. You need one for each processor, the bottom case, keyboard, LCD. You can download a widget called iStat Pro which will show you all the temperature readings from the sensors so that if there was a bad one, you could isolate it and then replace that one. Another thing is the fans will always be on. Do you think that they are sounding loud? Sometimes all you need to fix a loud fan is to pull the fan out and clean some of the dust out of the heatsink and fan area. You may also need to relubricate the fan post if it is vibrating. You do this with some silicon lubricant (WD-40 IS NOT A LUBRICANT) that can be picked up at a bike store or hardware store. How loud does your fan sound and what does iStat show as the fan's RPM?