Car running too cool.

Replaced water pump, thermostat, hoses, flushed system. Car always ran a little cool in summer but heat was hotter and thermostat said it was running warmer. Now in winter it is running just above "C" and very little heat. I blocked off 3/4 of radiator and still running very cold. I did notice the cooling fan started running when just one line above "C"then temp would drop and fan would shut off. Could it be Coolant Temp Sensor, Temperature Switch or Relay for Radiator fan motor. In what order would you replace or what else could it be? Thanks