Asus Crosshair IV Formula - Manual (Page 62)

7. Thermal sensor cable connectors (2-pin OPT_TEMP1/2/3)
These connectors are for temperature monitoring. Connect the thermal
sensor cables to these connectors and place the other ends to the devices
which you want to monitor temperature. The optional fan1/2/3 can work with
the temperature sensors for a better cooling effect.
Enable OPT FAN1/2/3 overheat protection in BIOS if you connect thermal
sensor cables to these connectors.
The thermal sensor cable is purchased separately.
2-36 Chapter 2: Hardware information