Mercedes X Klasse starting problem

That does sound frustrating. I had a similar issue with my old Vauxhall Corsa, though the symptoms were slightly different. The engine would crank endlessly, but never actually start. Eventually, it turned out to be a faulty key signal receiver. Is the security light behaving exactly as it should? Does it illuminate briefly when you turn the key to the 'on' position, then extinguish? Or is it flashing or staying on constantly? This would give us a clue whether it is really a car immobilizer problem.

The security light seems to be acting exactly as it should. It illuminates briefly when I turn the key to the 'on' position, then extinguishes. So I don't know if this is really a key signal problem or a car immobilizer issue. What did you do to solve the problem with your Vauxhall Corsa?

If the security light is behaving normally, that does complicate things. With my Corsa, a simple key programming resolved the issue. Given that your Mercedes is throwing error codes and the engine cuts out even after a brief start, it might be best to have a workshop check the key and immobilizer system. They have diagnostic tools that can pinpoint the problem more accurately than we can here. It is probably a car immobilizer problem.

I appreciate the insight. I took it to a shop, and you were right, it was a key programming issue. Apparently, the key signal was corrupted somehow. A quick reprogram and 45€ later, it's running like new. Thanks for the advice about the anti-theft system. Saved me a lot of time and potential headaches!