GLS Hybrid Air Leak Issue Resolved

I had a similar issue with my GLS, though I didn't have as many kilometers on it. The hissing sound is definitely a red flag. Have you checked the intake hose that connects to the throttle body closely? That was the culprit in my case. It's a common area for cracks to develop. Also, were you able to get any specific error codes when the check engine light came on? That might give you a more precise idea of where the leak is located.

Thanks for the tip! I'll give that hose another look with a magnifying glass. As for the error codes, I pulled a P0171 (System Too Lean) which kind of points to an air leak after the mass airflow sensor. Did your error code point to the same area?

Yes, P0171 sounds familiar. It definitely points towards an air intake issue. Given your symptoms and the error code, it’s almost certainly an intake leak somewhere. I would suggest taking it to a workshop so they can perform a smoke test to find the leak. It's the easiest way to find the exact location.

Appreciate the confirmation. I replaced the intake hose and it solved the problem. The total cost was around 100€. Fuel economy is back to normal, the hissing is gone, and the check engine light is off. Thanks again for the help!