After several years, the moisture will cause gap between the windshield and the IR receptor and the field of bubbles will become bigger and bigger until the sensor malfunction. I try to post a picture here to show you how the sensor works. In fact, the rain drop on the sensor area causes refraction that diverts all the IR away. The bubble in this area block the refraction and therefore the receptor always receive signal (IR) and assumes no rain at all. The reason why I do not want to change the windshield because it may damage my car (have you ever seen how the technician replaces a windshield - it is so appalling!). Unless there is a crack on my windshield.