B Klasse engine losing power

That sounds familiar! I had similar issues with my B Klasse: check engine light, major lag. The engine felt strangled. Have you noticed anything like a whooshing sound, almost like a compressor surge? It could be related to the boost controller or even a boost solenoid issue. Is it really bad turbo lag?

Yes, there is some turbo lag. I don't really know what compressor surge sounds like, but I think I might have heard something similar once or twice. Where exactly should I be looking for the boost solenoid or vacuum switching valve?

Honestly, I'm not the best at pinpointing locations under the hood. When mine was doing that, the sluggish engine and the boost problems, I took it straight to a workshop. They ran some diagnostics and found a defect in a vacuum switching valve. It's worth getting it checked out professionally, could save you a lot of headache!

I took it to the shop as you suggested. Turns out it was indeed a faulty vacuum switching valve, just like you thought. Repair cost me 140€. The car is running much better now. Really appreciate the help!