Twingo air intake leak

That sounds familiar. I had a similar issue with my old Opel Corsa a few years back. Same symptoms: increased fuel consumption, check engine light, and that tell-tale hissing noise. Have you checked the intake hose between the air filter box and the throttle body? That's where my leak was. Is the hissing constant, or does it change with engine RPM?

Thanks for the tip! I'll check the intake hose. When you say 'intake hose', do you mean the one right after the air filter, before it goes into what I think is the throttle body?

Yep, exactly that one. It's a common spot for cracks and splits, especially on older cars. Give it a good look over, and feel around it while the engine is running (carefully!). If you're not comfortable doing that, or can't find the leak yourself, I'd recommend taking it to a workshop. They can do a smoke test to pinpoint any air intake leaks. Could be something further down the line like the intake manifold gasket, but the hose is the easiest and cheapest thing to check first. Good luck!

Thanks a lot for the advice! I checked the hose and found a split. Replaced it, and the hissing is gone, the check engine light is off, and the fuel consumption seems normal again. Cost me 100€ for the part and the mechanic's labor. Appreciate the help!