This might void your warranty. It fixed the issue that one person was having after applying a firmware update however for this issue: iMac 27 SMC Firmware Update : Fans at maximum speed
“Ok, but be informed that you will do this at your own risk.
First download and install Refit. It’s an alternative efi for Mac OS X. Then you will have to download the SMC firmware on Apple Downloads and extract the content using Pacifist. In the package you will find 3 files, k23.smc (SMC firmware for C2D iMac late 2009), k23f.smc (SMC firmware for quadcore iMac) and SmcFlasher.efi (tool to flash the firmware). Put the firmware you need and the smcflasher in Users/yourname.
Then reboot you iMac. You will arrive on refit. Select the shell (command line icon). Then type the following :
SmcFlasher.efi -LoadApp k23f.smc
So the fs1: identify the hard drive
cd Usersyourname is the place you put the files
SmcFlasher.efi -LoadApp k23f.smc tell the system to launch the smcflasher and apply the firmware, in this case the SMC firmware for the quadcore iMac.
Reset tell the system to reboot.
Then you can check that the version of SMC is no more 0.000 and you can uninstall refit (see the readme file).”