Mini Cooper Diesel Engine Troubles

Whoa, sounds like your Mini is throwing a proper tantrum! The symptoms you're describing definitely point towards a turbo system issue, specifically with boost control. That whistling and hissing are classic signs of a leak somewhere in the system. Now, you mention an actuator fault... Is the pressure sensor working properly? Have you checked for any obvious cracks or loose connections in the hoses and pipes around the turbo?

Thanks for the quick reply! Regarding the pressure sensor, how would I go about checking that? I've visually inspected the hoses and pipes, and nothing seems obviously damaged, but I'm no expert.

Okay, think of it like this: your turbo is trying to inflate a balloon (your engine) but there's a hole in it. The pressure sensor is there to tell the car how full the balloon is. If the sensor is faulty, it's like having a broken gauge – it can't accurately report the boost pressure. Given the symptoms and your suspicion of the bypass valve, it's very likely that's your culprit. While you could try to diagnose the sensor yourself, honestly, with those symptoms, you're probably best off taking it to a reputable mechanic. Replacing that valve isn't usually too expensive or complicated, but they can confirm the diagnosis and rule out anything else. Don't keep driving it like that, you risk damaging the turbo system further!

Just wanted to give an update! Turns out you were spot on. The electrical bypass valve was indeed defective. Got it replaced for around 250€, and the Mini is back to its old self. No more check engine light, power is back, and that awful whistling is gone. Thanks a lot for your help!