Mini One Diesel Engine Trouble

That sounds very similar to an issue I encountered with my Mini One diesel. I had a recurring problem with the engine management light illuminating, accompanied by a noticeable decrease in engine performance. It turned out to be a faulty boost solenoid, part of the turbocharger system. This component regulates the boost pressure, which is essentially how much air is forced into the engine. When it fails, it can cause all sorts of issues. Have you considered checking the pressure sensor, bypass valve or the compressor surge valve? A malfunctioning bypass valve can also cause similar symptoms.

Interesting. I've heard of the boost solenoid, but I'm not entirely sure what it does. You mentioned compressor surge valve and bypass valve. Could you explain those terms a bit more?

Certainly. The compressor surge valve and bypass valve are both components designed to protect the turbocharger. The bypass valve, sometimes called a diverter valve, releases excess pressure when you suddenly close the throttle. This prevents 'compressor surge,' which is when the compressed air flow reverses and damages the turbo. As for whether it is the same issue as I had, it is difficult to determine, but given the symptoms are similar, the best course of action is to present it to a trusted professional workshop. The ultimate fix could be any of the components already mentioned including the pressure sensor.

Just wanted to give an update. Took it to a trusted workshop. They found a defect in the vacuum switching valve. The part itself wasn't crazy expensive, and the repair including labor was around 140€. Engine performance is back to normal now. Thanks for the help!