cpu fan not reflecting

working on a dimesnion 2350. machine when i opened up the case is close to be the worst ive ever seen for dirt. every intake totally covered with 1/4 of animal hair whole inside of box you could make a hair ball 5 inches in diameter and maybe this could be reason why cpu fan at boot says not found. cpu fan is connected to board and the fan is spinning just fine. have reset everything jumpers and battery. unfortunately in these dells there no way to turn off the cpu warning.

i know its not a worry cuz the fan is working but does anyone have an idea as to why the board is not reading the fact that it is working and connected.

 

The fan sensor on the motherboard has probably burned out. The Bios doesn't have an option to turn off the warning?

 

as bad as the inside of this machine was im surprised it didnt fry everything so not surprising that the sensor burnt out. running diags on it now but i have looked on other dell system before and i could never find a function to turn that warning off but when done with diags i will check again. i guess as long as everything else works after i redo it they will just have to hit f1 to continue if they want to use the machine. not that big a deal. thanks.