What are Rain Sensors?
How your car knows it is raining before you do
Simplified animation — not to scale.
In plain English
A rain sensor is a small electronic device usually hidden behind your rear-view mirror on the windscreen. It detects water droplets on the glass and automatically turns on your wipers at the correct speed — so you don't have to take your hands off the wheel or fiddle with controls.
"Imagine shining a torch through a clean window at night. If the glass is spotless, the beam bounces back brightly. Throw some water on the glass and the beam breaks up and dims. The sensor is watching that torch beam all the time, and when it sees the light scatter, it knows rain has arrived."
How it works
The sensor shines invisible infrared light through the glass at an angle. When the windscreen is dry, most of that light reflects back into the sensor. When raindrops hit the glass, they scatter the light and less comes back. The sensor measures this change and tells the wiper system to start working. The more rain that lands on the screen, the less light returns — so the sensor can increase wiper speed automatically as the downpour gets heavier.
Signs of trouble
- ⚠Auto wipers not switching on when it rains
- ⚠Wipers turning on randomly in dry weather or bright sunlight
- ⚠Sensor not responding to changes in rain intensity
- ⚠Wipers staying on full speed even when the rain stops
- ⚠Warning light on the dashboard related to the rain sensor
£70 to £230 total, depending on the vehicle
Always get a written quote. Prices vary by car, region, and parts brand.