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Mercedes Auto Lamp Function Inoperative

You get into the car and start your Mercedes-Benz. Next, you are greeted with “Auto Lamp Function Inoperative” error message on the dashboard. This problem is most often caused due to Mercedes-Benz light sensor malfunction.

What does this mean?

If you are getting Auto Lamp Function Inoperative  error it means that:

  • The low beam (headlights) will not turn on automatically.
  • Your windshield wipers will not turn on automatically if it rains.
  • Auto lights function may stop working.

It doesn’t mean that your lights won’t work. In fact, your lights will work fine when turned on manually, but they will not turn on automatically. Also if you enter a tunnel, your lights won’t turn on. You will have to turn the lights on using the light switch.

If it starts to rain, don’t expect the wipers to start working automatically. Use the wiper switch to turn on the wipers.

Common Cause

The most common cause that triggers the Auto Lamp Function Inoperative message is a defective Light and Rain sensors (LRSM).

Mercedes light rain sensorThis sensor is mounted at the top of the windshield (right in the front of the rear view mirror).

You can save money by purchasing the Rain & Light Sensor online and installing it yourself. The procedure is very simple and takes  less than 1 hour.

You may notice that the error goes away once the car warms up. This is because the light and rain sensor mounted at the top of the windshield returns to normal operation once the temperature in the car increases.

If your car is under warranty, take it to the Mercedes-Benz dealer to get it fixed free of charge. If the car is out of warranty you will have to get a replacement Light and Rain sensor that fits your car. The best thing to do is to remove the old sensor and get the part number from it. You are looking for a part number which will look something like A204 870 47 26.

Then go online and search for Mercedes Light Rain Sensor.

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