Mini Roadster Engine Issues

The symptoms you describe do point towards a boost leak somewhere in the turbo system. The whistling and hissing sounds, coupled with the engine light and reduced power, are indicative of that. Have you checked the intercooler hoses for any splits or loose connections? A faulty diverter valve could also be a culprit. It is difficult to say with certainty.

Thank you for your quick response. I inspected the intercooler hoses, but I did not find any obvious splits or loose connections. The diverter valve seems to be working correctly as well, but I have no possibility of measuring it electrically. Could it be something else related to the turbocharger, perhaps the boost pressure sensor itself?

Given that you've checked the hoses and diverter valve, and you suspect the boost pressure sensor, I would agree that is a strong possibility. The symptoms align with a failing sensor or its connections. While it could be other issues within the turbo system, the defective cable or plug connection would be my primary suspicion. Therefore I suggest you to have your car looked at in a workshop.

I would like to thank you for your assistance. You were right about the likely cause. It turned out that the cable connection to the boost pressure sensor on my Mini Roadster was indeed defective. The repair cost me 95€. After replacing the cable, the engine light is gone and the car runs smoothly again. I appreciate your help in diagnosing the issue.