Kix Intake Leak

That hissing sound definitely points to an intake leak. Given the increased fuel consumption and check engine light, the ECU is likely compensating for unmetered air entering the system. Have you checked the intake hose that runs from the air filter to the throttle body? Those are notorious for cracking, especially with age and heat cycles. Any codes being thrown? Knowing the specific code will help narrow down the possibilities.

Good call on the intake hose, I'll give that a really good look. I don't have a code reader myself. If that hose is cracked, could it really cause that much of a ruckus? I mean, the hissing is seriously loud!

Absolutely. Even a small crack in the intake hose can create a significant vacuum leak, especially when the turbocharger is spooling up. The ECU tries to maintain the correct air-fuel ratio, but with unmetered air entering, it'll dump in extra fuel, hence the poor mileage. That loud hissing is the pressurized air escaping. Honestly, with those symptoms, I'd bet money on a cracked intake hose. You could try a temporary fix with some silicone tape, but a replacement is the way to go. Best to get it properly diagnosed and repaired at a workshop to avoid further issues with the turbo.

Thanks for the confirmation. I checked the intake hose and found a split on the underside that I couldn't see before. Replaced it, cleared the code, and everything's back to normal. Parts and labor came out to exactly 100€. Appreciate the help!