Theres a plug under the radiator which is a trigger for the radiator fans, if you unplug it the fans will run continuosly.
check and see if the plug is connected or not, thats could be your problem..
Please be carefull doing this as the previous owner could have done this to avoid a minor overheating issue and as such is just a band aid fix..
so check you temp. regularly and see if it overheats after you have plugged the sensor back in and the fans are working like they should
cheers