RCZ Hissing Sound Air Leak

That sounds remarkably like the issue I encountered with my old Saab 9-3. I had similar symptoms: increased fuel consumption, the dreaded check engine light, and a hissing noise that drove me mad! It turned out to be a cracked intake hose. The hissing was the air being sucked in, bypassing the air filter and messing with the fuel-air mixture. Have you checked your air filter lately? A clogged filter can put extra strain on the hoses.

Thanks for the insight! I haven't checked the air filter recently, but I did replace it about 6 months ago. When you say 'intake hose,' are you referring to the one that connects the air filter housing to the intake manifold, or is there another hose I should be looking at?

Yes, indeed! I was referring to the hose that connects the air filter housing to the intake manifold. In my case, the crack was quite small and difficult to see without really bending the hose and looking closely. Given the symptoms, I would highly recommend having a workshop run a smoke test on the air intake system; that's how my mechanic discovered the issue. It's probably worth having a professional look, just to be certain. They will check all possible sources of leaks in the intake system.

Alright, that’s a great suggestion. I'll take it to a mechanic for a smoke test to pinpoint the leak. It was indeed the intake hose! A small crack was causing all the problems. Replaced it for 100€ and everything's back to normal. Thanks again for the help!