Cpu Fan header doesnt work

a few things come to mind.

First, whenever doing a new build always reset bios/cmos/uefi first to clear out any wierd setting that may have been introduced.

Seccond, check and see if your bios is set to turn the fan off when it is not needed. There are some boards that will allow for this, which is a nice and neat feature, especially if you have a nice big cpu cooler that can passively cool when just browsing the web.

Third, try a different fan. CPU fans have electronics in them, and while rare, they can go bad. Try the fan from the stock cooler on the 4pin and see if it behaves correctly. That 4th pin is an RPM signal that tells the fan what to aim for, (which is why it will always go to max when plugged into a 3pin fan header no matter what settings you give it).

If all that fails then I am afraid it is a bad mobo, and you will have to RMA. Kind of a dumb thing to go wrong, but I had a similar problem on my recent upgrade where case fan 1 was not working properly. I would have ignored it, except that I then had issues getting enough power to all 4 dimms of ram, so it was rather obvious my board had some defective power management. Cant wait for the new one to arrive tomorrow!

ok i tried a 3 pin fan to plug into the 4 pin fan header but it doesnt work either, even though it has the kick for a split second + there is voltage, as i checked with a voltage meter, i have smart fan setting but i turned that off, so the only thing im guessing is the mobo.

or is it the heatsinks fault ?

it could be you have rpm set in bios instead of pwm. or vise versa.
if you have an rpm fan then setting bios to pwm would have the results you describe of the fan starting and stopping. as pwm uses pulses to regulate the fan speed while rpm uses voltage variation. so give that a check in bios.

Solved it was the cpu fan header that was defective, i gave tge motherboard and i got a new one, anyway thanks for trying to help !