I have a 2011 27 iMac (2.7, 12GB, 2TB HDD + 250GD SSD Fusion drive). A week ago the PSU failed, which I replaced yesterday. Everything was fine for over 24 hours until suddenly in the middle of me using the iMac, the CPU fan suddenly ramped up to full-speed - and stayed there. The fan speeds up at the login screen phase of the boot process. I've tried SMC resets and NVRAM resets with no luck. I've got the fan under control using "Macs Fan Control" but obviously I would like to know what's wrong. I've installed iStat Menus and can't see anything obviously amiss with the readings. I've attached a screenshot when the fan control software is disabled and the CPU fan is speeding away. Could this be related to the PSU replacement? To reiterate, everything ran perfectly for over 24 hours after I fitted the new PSU. Activity Monitor shows no weird processes using lots of CPU. The HDD is the original (I know that replacing these with non 'official' drives causes the fans to run at full speed). Any ideas or advice would be much appreciated.