follow the wires from the fan to the engine block.there you should find a temperature sensing device screwed into the block it self Unplug the connected wire ,unscrew the sensor,and replace. You may need a special sized socket sold at auto parts stores if you do not have clear access to itwith an adjustable wrench or pliers. Usually,when you stop for a long redlight,or sit with the engine running,the coolant slows down in it's journey thru the system.This causes the coolant to rise in temperature and for the temperature sensor to tell the fan to turn on. The thermostat is located between the radiator and the motor and stays closed when you first start your engine to allow the coolant to run through the engine by way of a bypass hose.Once the coolant begins to warm up it affects the thermostat ,which then opens, and allows the coolant to circulate thru the system.As the coolant passes thru the radiator it is cooled by incoming air from the front of the vehicle.It then continues on it's way back to the engine to pick up more heat from the mechanical parts and continues it's loop. The thermostat does however control the coolants heat. If a thermostat does not open,it can cause the vehicle to overheat.If it opens too soon the coolant will never reach a good operating temperature. Hope that helps.