Hi,
Try these suggestions:
1. Try clearing CMOS using the 'Clear CMOS' jumper on the mainboard (You should see writing above the jumper block saying something like 'CLR_CMOS'
2. Update the BIOS (Download it from the Dell website)
3. Try and swap the chassis fan & CPU fan connectors around.
4. Load setup defaults in BIOS.
5. If all this fails, you may have a faulty thermal sensor on the replacement board.
Cheers, Michael.