Mercedes SLC auxiliary battery issue

Sounds like a classic auxiliary battery problem. Modern Mercedes often use a second, smaller battery to power auxiliary systems like the alarm, especially during start/stop events or when the main battery voltage dips. A discharged or aging auxiliary battery can definitely trigger those symptoms. Have you done a battery check recently, focusing on the auxiliary battery specifically? A multimeter can give you a rough idea of its voltage, but a proper battery load test is more definitive. Also, check the battery terminals and connections for any corrosion or looseness. Even a slightly poor connection can cause issues. If the battery is original, five years is about the lifespan I'd expect, so replacement might be necessary.

Thanks for the quick reply. Where exactly is the auxiliary battery located? And is a battery load test something I can do myself, or does it require special equipment?

The auxiliary battery location can vary a bit, but in the SLC, it's often found in the trunk, usually behind a side panel or under the floor. Consulting your owner's manual should give you the precise location. Regarding the load test: while you can measure the voltage with a multimeter, a load test requires a specialized tool that applies a significant electrical load to the battery to simulate real-world conditions. This reveals whether the battery can maintain voltage under stress, which a simple voltage reading won't tell you. Most auto parts stores offer free battery load testing services, so that's a good starting point. Given the symptoms and the age of the battery, and considering the error memory, it's probably best to have a workshop check it out.

Thanks for the advice. I took it to a workshop and you were right. The auxiliary battery was the culprit. A battery check confirmed it was discharged due to ageing. Cost me 330€ to get it sorted. Appreciate the help!