Question CPU fan not spinning on boot, but spins if I disconnect and reconnect it to the motherboard header ?

[Flawnex]

Hello,

As the title suggests, the CPU fan does not spin on boot. The BIOS warns about CPU fan speed not being detected, and I can get to Windows if I skip the warning by going through BIOS.

At first I assumed my CPU cooler fan was completely broken, but to my surprise it started spinning normally when I plugged it into a SYSFAN header. Then I tried the CPUFAN header again and it also worked.

I checked my fan settings in BIOS and it does not detect the CPU fan speed even if it is spinning. The fan doesn't exist according to BIOS but should spin if it did.
I have the stock Intel cooler for an i5-7400.

What could be causing the CPU fan to not start spinning on boot, and is there a way to fix it without buying a new one?

 

[Lutfij]

Never disconnect and reconnect devices while they are powered on and operating.

Make and model of your motherboard and PSU(and it's age too)? BIOS version for said motherboard? Try and source another fan/cooler that's PWM controlled and see if the issue persists on said CPU_FAN header.