Just finished a bit of work on my parents' iMac(Early 2009 24" 2.66GHz). I replaced the hard drive with a new 2TB hybrid drive and put new thermal paste on the CPU. Reassembled everything and booted from a USB drive to install. Immediately upon powering on the fans began blowing at full. I formatted the new drive and installed Yosemite. Rebooted and still fans are blowing. Looking at Macs Fan Control, only the CPU fan is misbehaving, running at max. I can however manually control the fan. CPU temps are fine at 33°C and do react normally when put under load(I ran a Geekbench test to see). I've tried rebooting, a PRAM reset, an SMC reset... not sure what to try next ???