When the CPU fan on the motherboard spins briefly and then stops, several potential issues could be at play.

Firstly, a power related problem might be the culprit. The fan could be receiving an inconsistent power supply. This could be due to a loose connection between the fan and the motherboard's fan header. Over time, vibrations or improper handling of the computer can cause the fan connector to become loose. Additionally, there may be an issue with the motherboard's power regulation for the fan header. If the motherboard is damaged or has a faulty circuit in the fan power section, it might not be able to provide a stable voltage to the fan, resulting in the fan starting and then quickly stopping.

Another possible cause is a mechanical problem with the fan itself. The bearings in the fan may be worn out or clogged with dust. Worn out bearings can cause increased friction, making it difficult for the fan to keep spinning. Dust accumulation can also impede the movement of the fan blades, causing the motor to struggle and eventually stop. Moreover, if the fan blades are bent or obstructed by something inside the case, it will affect the fan's normal operation.

Software related issues can also lead to this problem. The BIOS settings of the computer may be configured incorrectly. Some motherboards allow users to set fan speed profiles based on CPU temperature. If these settings are misconfigured, the fan may not receive the proper instructions to keep running. For example, if the temperature threshold for the fan to operate continuously is set too high, the fan may start briefly but then stop because it “thinks” the CPU temperature is within an acceptable range.

To troubleshoot, first, check the physical connections of the fan. Ensure that the fan is firmly plugged into the correct fan header on the motherboard. Clean the fan to remove any dust that may be causing mechanical issues. If the problem persists, enter the BIOS and review the fan settings, making sure they are appropriate for normal operation.