I have this problem. My BIOS have two operational modes for CPU FAN: Smart Fan Control On and OFF.
When this option is set to OFF, CPU Fan itself should start working as soon as CPU is heating up BUT that doesn't happen.
When this option is set to ON then BIOS itself decides when to switch on or off the CPU fan.
Gigabyte engineers told me that Smart Fan Control should be set to OFF QUOTE(gigabyte.com.tw)The CPU smart fan control in the BIOS was set to disable as default because we have to meet AMD’s requirement, only set this option disable could get AMD boxed fan work in smart fan mode.
But when I disable this option (i.e. set it to OFF) my CPU fan never turns ON! (that actually means that AMD Semprons CPUs don't have a thermal probe ...)