I have an iMac 22" 2011 whose fans blare up full speed in Yosemite, the problem disappears when I disconnect the internal audio cable from the logic board, this cable connects the logic board to mic/headphones connectors on the back. I tested it with the ASD OS test and 7 sensors on the logic board fail when the audio cable is connected. When I disconnect that cable, all is fine except there's no audio from the internal speakers (and possible the mic isn't working). However, the speakers do the startup sound when the machine starts. Also, the sound test in the ASD OS test is passed successfully. Which seems to indicate there may be some way in OS X to enable the sound despite the audio cable not being connected. Any idea how I could do this?