Yesterday when I turned on my imac the fans started spinnig at max speed (65535 RPM) and continued the rest of the day.
Today when I turned the computer back on the same problem accured. I have tried instelling software like fan control and SSD Fan Control but they doesn't seem to work.
Is there an other way to fix this problem?
Welcome to Apple Support Communities
First of all, open Activity Monitor (in /Applications/Utilities), choose "CPU" and tell us if there is/are apps that are taking a lot of CPU.
If there's nothing wrong there, the SMC manages fan speeds, so try resetting it to factory settings -> http://support.apple.com/kb/HT3964
If it does not work, maybe there's a broken sensor or fan motor on your iMac, so you need to run a diagnostic test to check it out. Depending on the age of your Mac, you have to use Apple Hardware Test (run an extended test) or Apple Diagnostics. Tell us the error messages they give, if any.
Fans keep spinnig at max RPM
