The DX79SI and DX79SR boards have an extra temperature sensor that allows the monitoring of a temperature up off the board. A cable with a temperature sensor on one end is provided. It plugs into a 2-pin header on the board. The sensor can be located wherever you feel it is needed (limited only by the length of the cable). For example, you could put it on the exterior of a SSD/HDD and then use this temperature in fan speed control decision-making (while sensors inside drives can be monitored, they cannot be used in fan speed control decision-making).
If the cable is missing or broken, a reading like -55 is received. The sensor is disabled and not displayed if, at the point when Intel Desktop Utilities (IDU) is installed, the cable is detected to not be present. If you had the cable installed originally and later removed it (or broke it), the sensor will continue to appear in IDU (with bad reading). If you want to remove the display of this sensor, uninstall IDU, reboot and then reinstall IDU.
Hope this helps,