Rain Sensor

It sits behind the windscreen near the rear-view mirror, often combined with a light sensor. It is coupled to the glass with a gel pad. Sensitivity is set with the wiper stalk. Many cars also use the sensor area for automatic headlights, humidity sensing to prevent misting and the camera for driver assistance.

Infrared light is reflected inside the glass; water drops change the reflection and the sensor detects them.

After windscreen replacement the sensor needs a new gel pad and correct mounting. Dirt or chips over the sensor can cause erratic wiping.