20'' Aluminium iMac, 2.4ghz, 4GB RAM, 2133. My internal hard drive recently fatally crashed out so I replaced the hard drive with a nice big 1TB one. Since i put the machine back together and reinstalled OSX (Leopard 10.5), this fan has been going constantly: The one you can see at the bottom there. I guess its the CPU I installed the iStat widget to check what the temperatures were like inside. I got this back: By this point I'm assuming the sensor/s is either disconnected or something worse, and the fan is being clever and safe by running all the time. So I open it up and check the connections which seem to be fine, but i give them a blow etc and THEN I got a reading on the temps, and the fan was working nice and calm. I closed her up, and typically, the problem started again. I assumed that the case had something to do with it, pressing on the sensor or something. Which it wasn't. I did all the things I did the first time again, but this time with no temperature readings at any stage. Now I'm not sure what to do. The fan is running at full speed all the time and I have no temperature readings, save the two shown above. I also tried the application 'Temperature Monitor' with the same results. Questions I have are: Which connection would be reading the temperature, so I can make sure its properly connected? Is there a way to reset the fans or something? A friend told me about something like that. And obviously, have any of you had this problem, and know how to fix it?