there are two fan sockets on the motherboard side to side i suppose if you were in the right hand not the CPU_FAN then it may give error. Check you CPU fan goes to the left side any system fans you have go to the right hand
If its in the right fan socket then in the bios area : Fan Control [Enabled]
Enable/Disable the control of the system/CPU fun speed by changing the fan voltage. Configuration options: [Enabled] [Disabled] - try disabling this function as this adjusts the fan speeds in regards to idle etc.
Cant think of anything else