Fan constantly running at 100%

"Hi @nixni ,

Try this and see if it works:

Power down the system

Remove the power cord from power supply. (power cord from PSU to AC wall outlet)

Remove CMOS recovery jumper(#17 in image below - note its’ connection)

Remove CMOS battery (#24 in image below) (note the orientation of the battery)

Hold power button for 5 seconds

Reconnect the power cord to the AC wall outlet.

Power On computer and then hold Power again to Power off

Connect CMOS jumper back the way it was

Replace the CMOS battery back correctly

Power up (system may beep twice)

Press F1 for BIOS

Set the BIOS to the default settings

Set time and date

Press F10 to save BIOS settings and exit

Check if the fan is still running at 100% continually. If it still does you have a motherboard problem, possibly the temp sensor thinking that the CPU is overheating but it could be something else ;-)

Image taken from p.72 of the service manual for your desktop. Click on image to enlarge for better viewing"