kajjot:
Actually it might not be the best website for car issues but it is the right thread and in the proper spot in the forum. Please look at sub forums as there is a whole beyond the keyboard section which has a car section.
OP:
The fan is controlled by a temp sensor. If you unplug it the fans should turn on which will tell you that the sensor is bad. You also might have an airlock. Temp sensors don't work in air so if there is an air pocket by the sensor it won't work.