Mercedes GLK diesel check engine light and power loss resolved

I had similar symptoms on my GLK. Check the bypass valve and associated vacuum lines. A pressure drop there will definitely cause turbo lag and reduced power.

Bypass valve, huh? Where exactly is that located, and how would I check it? Also, could a faulty pressure sensor cause these issues too? I'm not really sure where to start looking for a vacuum leak.

The valve is near the turbo, but honestly, chasing vacuum leaks without proper tools is a pain. If you're seeing a boost pressure fault consistently, and experiencing all those symptoms, I'd suggest getting it checked by a workshop. They can diagnose the vacuum system properly.

Took your advice and brought it to the workshop. It was a defect in a vacuum switching valve, just like the internet suggested. They replaced it, and now the GLK is back to its old self. Total cost was 140€, which isn't too bad. Appreciate the help!