CLK auxiliary battery issue

Sounds like a classic auxiliary battery failure. The CLK's electrical system relies heavily on that little guy, and when it starts to go, you see all sorts of strange things happening. That error message is a telltale sign. Have you checked the voltage of the auxiliary battery itself? A healthy one should read around 12.6 volts. If it's significantly lower, that's your smoking gun. Also, have you noticed if the problem gets worse in cold weather? That can also point to a failing battery. It's definitely worth investigating the charging circuit as well, but the battery itself is the most likely culprit given the symptoms.

Thanks for the quick reply! I haven't checked the voltage yet, but I will. Where exactly is the auxiliary battery located in the CLK? And is it something I can easily replace myself, or am I better off taking it to a shop?

The auxiliary battery is typically located either under the rear seat or in the engine compartment, often near the main battery. Check your owner's manual for the exact location in your model year. While battery replacement itself isn't terribly difficult (disconnect negative terminal first!), you're dealing with the car's electrical system. Considering the potential for further issues and the error codes you're already seeing, it might be wise to get it checked by a professional. They can properly diagnose the charging system too. Better safe than sorry when it comes to car electronics.

Thanks for the advice! I took it to my local shop, and you were spot on. It was the auxiliary battery. They replaced it, and the error message is gone, and everything seems to be working fine now. The total cost was around 290€, including labor and the battery. Appreciate the help!