Audi Repair: Coolant fan on my A3 Sport FSI 4 yrs old, temp gauge, sensitive switch

Question
Dear Jan - how is the coolant fan meant to operate?  Mine seems to be running all the time.  The last few days I have been driving my car on the motorway followed by driving it around London at no more than 30 mph and I can hear the fan is on all the time and even when just sitting at the traffic lights (the temp gauge is at 90).  The fan seems to be on a lot even when I am just driving around London.  Isn't the fan meant to run just when the engine is hot?  Yesterday the fan came on immediately when I started the engine and I had not used the car for 12 hours so the temp gauge is low.  Appreciate your help on this.  Thanks!

Answer
The cooling fan is supposed to come on when the temperature of the water in the radiator gets above a certain value, and this is regulated by a temperature sensitive switch.  I would check the operation of this switch, since they do go bad fairly frequently.  The fan sometimes also is connected to the air conditioning, so does switching the A/C off make a difference?  Let me know what you find,  Jan