Mercedes A Klasse Diesel Electrical Fault

I had a similar problem with my A Klasse. The error message and weak starts sound familiar. Check the auxiliary battery voltage. A significant voltage drop when you turn on the ignition is a strong indicator. What voltage do you see when the car is off, and what does it drop to during startup?

I don't have a multimeter. Where is the auxiliary battery located? Is it easily accessible, or should I take it to a workshop?

If you don't have a multimeter and aren't comfortable checking it yourself, take it to a workshop. It's probably the auxiliary battery. It might be hard to reach, depending on your model, and messing with car electronics without proper knowledge can cause further issues. Better safe than sorry!

The workshop confirmed it was the auxiliary battery. They saw the low voltage and significant voltage drop during startup. A new battery was installed, the error message is gone, and the car starts perfectly. Thanks for the advice!