Mercedes ML Diesel Auxiliary Battery Issues

That sounds familiar. Are you seeing a voltage drop? My ML had a similar problem, and the auxiliary battery was the issue. Have you checked the battery's voltage? Is it charging correctly? What's the voltage when starting the engine?

I haven't measured the voltage during engine start. Where is the auxiliary battery located? What voltage should I expect from a good battery?

It's usually under the passenger seat, but check your manual. A healthy auxiliary battery should be around 12.6 volts when the car is off. Anything much lower indicates a problem. If the voltage is low, especially when starting, the auxiliary battery is likely the problem. It might be time to replace it. If you lack the tools or know-how, a workshop visit is best.

The auxiliary battery was indeed faulty. I had it replaced at a workshop for 290€. The error messages are gone, and the engine starts normally now.