|The cpu fan wiring MUST be connected, properly, to the cpu fan header on the mboard. |
The cpu fan must have an rpm output - it has to have at least 3 wires wired up to the cpu header.
You can use a 3 wire fan wiring connector on any 4 pin cpu fan header, or a 4 wire cpu fan wiring connector on any 3 pin cpu fan header, as long as you connect it to the proper pins.
See this and make sure the wiring from the cpu fan is connected properly.
Motherboard (CPU) 4 Pin Fan Connector Pinout
The pins on the header are closer to one side than the other, and so are the holes in the female wiring connector from the fan, and the female wiring connector from the fan usually has external ridges that line up with a plastic tab on one side of the header, so the 3 or 4 wire female wiring connector can normally only be installed one way, but it's quite common for the tab on the mboard header to be broken off, and you can install the female connector backwards in that case, or you can install a female connector with 3 wires/holes on the wrong three pins of the 4 pins.
Unlikely - in some bioses you may be able to specify the minimum rpm of the cpu fan. In that case, make sure the minimum rpm is a little under the current reading for the fan's rpm shown there or elsewhere in the bios , or lower, Save bios settings.
If it's a third party fan, e.g. it didn't come in a boxed set with the cpu, sometimes the fan won't be detected by the bios properly if it's rpm is lower than the range of rpms the bios defaults expect - the only cure for that is to get a CPU fan the bios does recognize properly. If the current rpm reading for the CPU fan in the bios is shown as 0, and if every requirement has been met, you have that problem.
If you have updated the bios version, and it's only after that you got this message, go into the bios and load bios defaults, Save settings ! You are always supposed to do that after changing the bios version, because the Cmos contents for the changed bios version are often NOT auto loaded when you flash the bios, and in that case the bios may NOT work properly until AFTER you have loaded bios defaults. (OR, when the system has no AC power to the power supply, you could move the clear Cmos jumper on the mboard to the clear position , move it back, or remove the cmos battery on the mboard, then re-install it - + must be visible when it's installed - but in that case, you will get a "Cmos Checksum Error" or similar message while booting and you will have to go into the bios Setup and set the Date and Time, Save bios settings.)