XC70 Diesel Engine Power Loss

That sounds frustrating! I had a similar problem with my 2018 Renault Megane, although it was a petrol engine. The symptoms you describe – reduced power, check engine light, and that whistling sound – were almost identical. In my case, it turned out to be a faulty turbocharger pressure sensor. Have you checked yours?

Thanks for the input. Where exactly is the pressure sensor located, and is it something I can easily inspect myself? Is there a way to check it without specialized equipment?

On the Megane it was located near the throttle body, relatively easy to spot once I removed the engine cover. You might want to check online for the XC70 specifically, maybe on some parts diagram. Sadly, without diagnostic tools, like an OBD reader and some know-how, it's hard to test it. Given the symptoms, especially the whistling noise and stalling, it might be a defect in the wastegate valve. I had the same hissing, because pressure was not properly regulated. Ultimately, I had to take it to a workshop to get it properly diagnosed and repaired. They had to replace the whole turbocharger unit on mine.

Okay, I appreciate the detailed feedback. It sounds like this could be beyond my capabilities to properly diagnose and fix. I'll take it to a trusted workshop and have them take a look. They confirmed it was a defect in the wastegate valve and quoted 860€ for the repair.