Sprinter gasoline engine boost pressure issue

That sounds rough. I had something similar happen with my old Vauxhall Astra, though it was a different engine. I also had a pressure drop issue linked to the boost controller. Is it throwing any specific codes? I'd start by checking all the vacuum lines for cracks or splits, as a tiny leak can cause big problems.

When you say vacuum lines, do you mean the little rubber hoses connected to the intake? Where is the boost controller usually located? I'm not very mechanically gifted, so please explain it simply.

Yes, those little rubber hoses. On my Astra, many were brittle and cracked, causing my pressure sensor to give wrong readings. The boost controller's location depends on your engine setup, but it's usually near the turbocharger. If you aren't super confident, it might be worth getting it checked at a workshop before you cause more damage.

I took it to a workshop. It was a defect in a vacuum switching valve, just like I suspected. The repair cost was 140€. It runs like new now.