Peugeot 308 boost issue

Sounds like a boost leak somewhere. A whistling or hissing noise under acceleration is a classic symptom. Reduced power and poor engine response would also point to that. Have you tried checking all the hoses and connections related to the turbo? A faulty wastegate valve could also cause these issues. Does the car feel like it has turbo lag, like the power comes on late or not at all? You should also check the throttle body for dirt and proper operation.

Thanks for the reply! Yeah, it definitely feels like there's turbo lag, like it's struggling to build boost. I'll double-check all the hoses and connections tomorrow. When you say 'wastegate valve,' is that something I can easily inspect myself, or is it buried deep in the engine?

The wastegate is usually attached to the turbocharger, so it can be tricky to get to depending on the engine layout. Given all the symptoms you're describing – the whistling, the lag, the stalling, and the increased fuel consumption – a faulty wastegate is very likely. It's probably stuck open, preventing the turbo from building proper boost. Honestly, with those symptoms, and without specialist tools, it's best to take it to a workshop. They can do a proper smoke test to find any leaks and diagnose the wastegate properly.

Thanks, appreciate the advice. I took it to the shop and you were spot on. It was indeed a defect in the wastegate valve, and they ended up replacing it. Cost me about 860€, but the car is running perfectly again. No more check engine light, the power is back, and the hissing is gone!