Seat Mii Engine Hissing and Power Loss

Whoa, that sounds frustrating! The symptoms you're describing, the whistling and hissing, combined with the engine light and power loss, it rings a bell. I had something similar happen with my old Opel Astra a while back. It turned out to be a faulty mass air flow sensor. Check if the automotive sensor is covered under any warranty. Could also be a vacuum leak somewhere in the air intake system. Any temperature fluctuation readings?

Interesting! Mass air flow sensor, you say? That's a different automotive sensor than I was expecting. Where exactly is that located in relation to the charge air temperature sensor? And what kind of temperature fluctuation were you seeing? My reader only gave me the generic error code.

Okay, so the mass air flow sensor measures the amount of air entering the engine. This is slightly different to the charge air temperature sensor which measures the temperature of the air after it has been compressed by the turbo (if your car had one). Since you've already got the error code pointing towards the temperature sensor, and you're hearing hissing, I'd stick with your initial thought. A defective cable or plug connection is common. Honestly, for 95€, I'd get it sorted at a workshop unless you're super confident with electrics. The loud hissing when accelerating might suggest a leak which should be repaired to avoid further damage.

Just wanted to let you know: it was the charge air temperature sensor! The cable and plug connection were defective as suspected. 95€ later, the Mii is running smoothly again. Thanks for the advice!