97 Subaru outback main fan engages and quits is it the fan or thermosensor?

Electric fans are not made to run constantly unless the temperature dictates it. Once the coolant gets to a certain temp, the Coolant Temp sensor will send a signal and a relay will trip sending current to the fan to turn on, once below that temp, the fan should quit. Start your car when cold and see if the fan turns. It shouldn't because you rcoolant is not up to temp so why waste the energy to cool somethign not needing it. Now if you fan does NOT turn on at all.....then that is a concern. We are so used to old style mechanical fans that ran all the time.....technology takes us forward even if we go fighting what we knew from long ago. If your fan is switching off and on rapidly then yes have it checked,.........I would check the relay first