Seat Leon Diesel engine losing power

Oh, that sucks! Check engine lights are the worst. Sounds like your turbo's having a rough time. Is it happening all the time or just when you put your foot down? Could be a dodgy boost controller messing things up. Is the intake manifold making weird noises? Compressor surge can sound pretty scary. I had similar issues on my old ride, turned out to be a cracked hose causing a massive vacuum leak.

How can I check the boost controller and intake manifold myself? I'm not super mechanically minded, but I'm willing to have a look before taking it to a garage.

Yeah, poking around might reveal something obvious, but messing with the turbo system without knowing what you're doing can cause more harm than good. If you're seeing reduced engine power and the engine light is on, it's likely the ECU is pulling back the boost to protect the engine. Given the symptoms, especially the poor engine response and higher fuel consumption, I'd say get it checked by a pro. A faulty vacuum switching valve sounds plausible, especially if it's affecting the boost pressure. Turbo lag can also increase.

Took it to the shop, and you were spot on! It was a faulty vacuum switching valve. They replaced it, and the car runs like new again. Cost me 140€. Thanks a million for the help!