I would reseat the connection for the sensor to the master I/O board. If the problem continues, call dell and have them send a tech with another thermal sensor and master I/O board.
PN: FT231 Thermal Sensor Assy
PN: TG003 Master I/O Board
I had the same problem with my XPS 730, I have had it for less than 2 months. Wake up one morning and got the same error message. There were not lose cable in my case. I chat with support and they are going to send me a tech with the new* motherboard. I did nothing out of the ordinary the night before so I have no idea why i suddenly got that error. The major systom is that the fans are now going crazy, it almost unbearable.
*new to me
Yes Chris in that Order... Sensor failure then noisy fans. I don't think it could have happened the other way around. The computer was off for about 8 hours before that happened.
Now if the motherboard is not the issue then i'll rather keep my current board instead of replacing it with a refurbished board. The Dell tech support did not know where the temperature sensor is either, he was searching throught the same manual that was shipped with my system and it's not detailed on the manual. I would have thought that the Dell people would have had access to more detailed information about their systems than what is available on line to the public.