Seat Exeo Hissing Sound and Check Engine Light

I had a similar issue with my Exeo. A loud hissing sound under acceleration and the check engine light were the giveaways. Have you checked the intake hose running from the air filter to the throttle body and turbocharger? That's a common spot for leaks. You can try a simple leak detection test by spraying some carb cleaner around the hose while the engine is running and listening for changes in the engine's idle. Be careful though, carb cleaner is flammable.

That's helpful, thanks. When you say intake hose, are you referring to the corrugated plastic one right after the air filter box? And did your check engine light also come on intermittently at first, or was it constant?

Yes, I was referring to the corrugated plastic hose. Mine was cracked on the underside, so it was hard to see at first. My check engine light was intermittent for a while, then it became constant. If you're not comfortable doing the leak detection yourself, I'd recommend taking it to a workshop. They should be able to diagnose it quickly and give you a precise estimate.

Thanks a lot for your help. I took it to a workshop, and as you suspected, it was indeed the intake hose. They replaced it, and the total cost, including labor, was right around 100€. Fuel consumption is back to normal, and the check engine light is off.