The CPU fan is provided to prevent the CPU from overheating. You may want to know if you CPU fan is spinning fast enough to keep the CPU in a cool condition. CPUs, in general, have a "maximum bearable temperature," which means that once the CPU temperature exceeds this, the computer may begin to freeze occasionally or restart without any warning. Extreme temperature may also reduce the life span of your CPU.
Reboot your computer and wait for the "BIOS prompt" to appear. Press the key that corresponds to the BIOS before the computer fully loads the operating system. Keys are usually "Del," "F10" or "F6," although this may differ, depending on the kind of BIOS you are using. If the computer boots too fast so that you cannot read the instructions on how to go to the BIOS, press the "Pause" key to stop the screen for a moment, and then "Break" to resume.
Navigate to the "Power" tab (or something similar) in the BIOS screen, and then select "Hardware Monitoring," "System Health," "PC Health Status" or something similar. You will see the speed of the CPU fan (usually measured by "RPM"), as well as the CPU temperature.
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Check if the motherboard installer has an "extra software" that allows you to monitor your CPU's temperature. This is if the BIOS you have installed does not monitor the speed of the CPU fan. Insert the motherboard installer into your optical drive, and check if it has this kind of software. If it does, install it to your computer, and launch the program to check the speed of your CPU fan.