here is what happens: I shut down the OS (Windows 7 Ult), and then switch off the power supply (yes, at the back of the computer). (The BIOS mode is "power on after power failure"). When I switch the computer back on later (e.g. after a couple of hours or the next day), sometimes the CPU fan keps running at full speed. I also found out that, if this happens, the thermal margin (as seen on IDU or in BIOS) is 0, so indicating that the CPU is at its maximum temp. However, the cooling block on the CPU does not feel hot, so it seems that the CPU temp sometimes is read wrongly.
If I enter the BIOS as soon as I detect the fan does not slow down after power up, BIOS states in Real Time HW monitoring: Processor Thermal Margin: 0, Internal Temp 35 C, Remote Temp 33 .
After exiting the BIOS (Exit-Save changes), processor fan does slow down.
Thanks for giving the detail. The reason why I ask the question is that I have to ascertain if your situation is similar to what I observed in my unit. Indeed it is the same. Others have also affirmed such a condition in other threads of the forum. I believe that such a situation is hardware related and it is not related to a particular mobo model. What I did is to just leave the unit plug in and the power switch at the back always in the "on" position. The switching "on" and "off" of the system is through to the OS only. In this way, the condition you described no longer recur.