Almost all modern cars have plenty of sensors. The light and rain sensors are important for monitoring the weather and adapting the car to suit. They are usually found together on the windscreen near the top by the rearview mirror. If there is a problem with them, then the warning light will show up.
The sensors explained
The rain sensor is a switch activated by rain. It is responsible for automatic windscreen wipers and protecting the internal systems from the rain. It works by bouncing an LED light off the windscreen, which becomes distorted by water on the surface. The light sensors work in a similar way, detecting the amount of light coming through the windscreen. If it is below a threshold then it automatically activates headlights while the car is on.
The photoelectric ‘light’ sensors activate the headlights during low lights, although they only do this when first switching the car on, as many drivers prefer to manually toggle the headlights while in motion.
There are limitations to both sensors, as different conditions are difficult for a computer to understand. The rain sensors can be thrown off by debris or dirt on the windscreen which alters the amount of light reflected back, the dirt can also affect the light sensors which are blocked off from any light. This can be remedied with a squirt of windscreen wiper fluid and a quick wipe across the windscreen. If the light persists then you may want to have a car wash. The light sensor is also thrown off by fog or intense rainfall since light can still be detected.
What the rain and light sensor warning light means
The light may vary across vehicles, but the image should generally be the same, you can check in your owner’s manual if not. An issue has been detected with either the computer or the system and will be deactivated. A diagnostic testing code will be saved to your computer for a mechanic to identify the issue more easily.
You can still manually use your headlights and wipers. Reasons for the sensors to fail can be due to a crack/chip repair or a replacement non-factory windscreen.
Is it safe to drive with the rain and light sensor warning light on?
The rain and light sensor warning light indicates that there is a problem with the sensor, which is used to automatically turn the headlights on and off, as well as the windshield wipers. Your headlights and wipers should still be working manually, you just have to remember to switch them on and off. Relying on automatic systems can leave you in trouble when they decide to stop working. You must be prepared to take control of the vehicle in all situations.
How do rain-sensing wipers work?
Rain sensors are designed to detect moisture on the windshield and activate the windshield wipers automatically which is considered a beneficial safety feature to have especially through heavy rain and dark motorways.
The sensing element usually located behind the rearview mirror works by sending out a beam of infrared light – the more water on the windscreen, the less light is reflected. The control unit is typically located behind the dash and is connected to the sensing element by a wire.
How do headlight sensors work?
Vehicle headlight sensors are designed to turn the headlights on automatically when it becomes dark outside. Most headlight sensors are located near the centre of the vehicle at the top of the dash. The sensor uses a light-sensitive resistor (photoelectric) to detect the amount of light present. When the amount of light present drops below a certain level, the headlight sensor triggers the headlights to turn on. It’s important not to always rely on this feature as in some conditions this may not be as effective as you’d imagine such as fog or rain wherein you’d need to turn headlights on to aid visibility.
Happy driving!