cooling problems


hi all,

i have a problem with my charade gtti cooling system, hoping that i can get some help here....
the temperature indicator in the car will indicate cold coolingfluid during normal highwaydriving, the heater will blow hot air though. if i start the car and leave it running idle or drive the car under conditions where it doesnt get enough cooling, the fan wont switch on cause the temperature indicator wont reach the temperature that is needed to turn on the radiator fan.
the odd thing is, 2 days ago the tempindicator did reach the switchtemperature, and the fan did switch on.

is it the temperaturesensor of the coolingwater that causes the trouble? if yes, where is it located?
if not, what could be the problem?