The sunlight sensor is used by the climate control system to allow for dual zone (upper and lower) heating or cooling, when the sun is shining on the car the large amount of glass and a small cockpit will change temperatures rapidly. When the sensor is in the sun the heater can direct warmer air to your feet and cooler air to the upper vents, counteracting the effect of the sun.In summer time the Air will remain in MAX cooling longer if the sun is present.