A Klasse alarm system malfunction

I understand your frustration. I experienced a similar issue with my Opel Corsa. The symptoms suggest a problem within the alarm control module itself. The error memory likely holds a clue. The fact that the alarm system is completely non-functional, despite the car otherwise operating normally, points away from simple issues like door sensors. Consider the possibility that the alarm system is coded to activate optical locking, that is, it attempts to signal via the indicators, but this function might be faulty, thus causing the error and system failure. This could be a software glitch or a hardware fault within the module. Have you recently had any electrical work done on the vehicle?

Thanks for the quick reply! Optical locking makes sense. No recent electrical work, but there was that power failure last week. Could that have messed something up with the alarm system's programming? And where exactly is the alarm control module located in the A Klasse? I'm trying to avoid a costly trip to the mechanic if it's something I can check myself.

The power failure could very well have induced a voltage spike that corrupted the alarm control module's programming, though it's difficult to say for certain without diagnostic equipment. As for the location of the alarm control module in your A Klasse, I am not familiar with that particular model, it is usually located behind the glove box or under the steering wheel column, but it would be best to consult a repair manual for the exact placement. Given the complexity of modern alarm systems and the potential for immobilizer-related issues, I would recommend having the system professionally diagnosed at a workshop equipped with Mercedes-specific diagnostic tools. They can read the error memory, re-code the alarm system if necessary, and ensure the immobilizer is functioning correctly. A faulty car alarm can create a plethora of problems, including preventing the vehicle from starting.

Thanks a lot for the advice! I took it to a workshop, and you were right. The alarm system was indeed coded to activate optical locking and needed to be reset. They recoded it and cleared the error memory. Cost me 45€, but at least it's fixed! Appreciate the help.