Twingo Turbo Whistle Power Loss

Those are concerning symptoms. A whistling noise, coupled with power loss and the check engine light, definitely points towards a turbo issue. The boost pressure actuator controls the turbo wastegate, which regulates the boost pressure. If it's faulty, it can cause all sorts of problems. Can you check the actuator arm for free movement? If it's stuck or very stiff, that would support your theory.

Thanks for the quick reply! When you say 'free movement', what exactly should I be looking for? How much play should it have? Also, what happens if it's not moving at all? Is that game over for the actuator, or is there a chance it can be freed up?

When I say "free movement," I mean the actuator arm should move smoothly and easily without resistance when you try to manipulate it. There should be a small amount of play, but it shouldn't be excessively loose. If the arm isn't moving at all, it's highly likely the actuator is seized or defective. While it might be tempting to try and free it up, if it's electrically controlled, the bypass valve might have failed internally. Considering the symptoms, and if the actuator arm is indeed stuck, I would recommend a visit to a trusted workshop to get a proper diagnosis and potential replacement. It's essential to rectify turbo issues quickly to prevent further damage to your engine.

Cheers for the advice! You were spot on. I took it to a garage, and they confirmed it was a defective electrical bypass valve. It cost about 250€ to get it sorted, but at least the Twingo's back to its old self. No more whistling, stalling, or excessive fuel consumption!