Winbook N5/Dell 2650 Fans stop working

The cpu (jp16) and back panel fan (jp20) both stop running after a few seconds. I had this problem repaired under warranty in 2004, the problem happened again after a while. There is no warranty now. Can I replace the faulty (sensor?) myself. I have the laptop stripped down in case the MB needs replaced. There are red, black and white wires going to the MB, what would the voltage be supplied to each. One would be 5v. I'd like to repair the laptop if I can it works well. At present I have two large cooling fans behind and a usb powered fan/base unit it sits on, but this is a hassle.

Thanks for any help.
Colin