It is an optical sensor, not a moisture sensor
The sensor shines infrared light into the windshield at an angle. With dry glass, that light reflects internally and comes back to a detector at full strength. Water droplets on the outside scatter some of it away, so the returning signal drops — and the amount it drops tells the car how hard it is raining.
This means the sensor is reading the glass itself, which is why it is fussy about optical contact.
The gel pad
The sensor cannot touch glass directly — an air gap would ruin the optical path. A clear silicone gel pad couples it to the windshield.
That pad is single-use. Reusing a pad from the old glass leaves microscopic air bubbles, and the result is wipers that trigger randomly, wipe constantly in dry weather, or never trigger at all. A new pad costs very little; forgetting it is one of the most common shortcuts in this trade.
The bracket is part of the glass
The mount holding the sensor is bonded to the windshield at the factory, and it is not transferable. Glass ordered without the bracket cannot take your sensor, which is why the year, make and trim matter when the part is sourced.
Need this done rather than explained? There is no shop to drive to. We come to wherever your car already is — your driveway, your workplace car park, an apartment lot, or the side of the road it stopped on — anywhere in greater Sacramento, with no call-out fee and no mileage charge. Give us the year, make and model and you will have a firm price before the call ends.