Scenic Boost Pressure Issue

Whoa, that sounds rough! I had something kinda similar happen with my Scenic. The hissing and power loss are definitely worrying. Have you checked all the hoses connected to the turbo? A boost leak can cause those exact symptoms. Sometimes it's just a cracked hose or a loose clamp. It's a cheap and easy thing to check before you start suspecting the boost pressure sensor itself. Is the whistling constantly there, or just under load?

The whistling seems to happen mostly during increased engine load, especially when accelerating. Could you explain where the hoses are that I should check? I don't know much about the turbocharger system. By 'under load', do you mean when the turbocharger is actively engaged?

Yeah, 'under load' means when the turbo is working to give you more power. Okay, so the hoses I'm talking about are the ones that connect the turbo to the engine. Basically, follow the metal pipe coming out of the turbo (it looks like a snail shell). It connects to rubber hoses. Check those hoses and their connections very carefully for cracks or looseness. Honestly, though, with all those symptoms, it might be best to get it checked out by a workshop. A faulty sensor can cause all sorts of weird problems, and they'll have the right tools to diagnose it properly.

Thanks for the help. I had a professional diagnostic assessment done. It confirmed a faulty boost pressure sensor, specifically a defective electrical connection. The repair, replacing the connector and wiring, cost 95€. The car now runs perfectly, with none of the old symptoms. Thanks again for pointing me in the right direction.