Seat Toledo Air Intake Leak

Whoa, sounds rough! I had a similar issue with my old Renault Megane, although it wasn't quite the same. Check engine light popped on, and my MPG tanked. Turned out to be a split in the intake hose after the air filter box, causing a massive vacuum leak. Made a crazy hissing noise too, especially under load. Have you checked all the hoses for cracks or loose connections? It's an easy DIY inspection.

Thanks, that's helpful. When you say 'after the air filter box,' do you mean the hose leading to the throttle body? I've visually inspected the readily accessible sections, but I might need to get a bit more hands-on. Any tips for spotting a hairline crack?

Yep, the hose leading straight to the throttle body. A small crack can be tricky. Try this: with the engine idling, carefully spray some carb cleaner or starting fluid around the intake hoses, especially near the connections. If the engine RPMs change, you've found your leak! Otherwise, honestly, with those symptoms, and if you're not super confident, taking it to a trusted workshop might save you some headaches. They can do a proper smoke test and pinpoint it fast.

It was indeed a cracked air intake hose, just as suspected. Replaced it, and the total cost was right around 100€. Fuel economy is back to normal, and no more hissing. Thanks for the help!