Seat Mii AC Issues

It appears you've encountered a perplexing scenario. Based on your detailed diagnostic account, I concur that a sensor malfunction within the HVAC system is highly probable. Have you verified the integrity of the wiring harness connected to the temperature sensor? A fractured wire or corroded terminal could mimic the symptoms of a defective sensor. Furthermore, is there any indication of a coolant leak in proximity to the heat exchanger? A diminished coolant level can trigger erroneous temperature readings. Finally, have you considered a complete system reset following the sensor replacement?

Thanks for the quick reply! Regarding your points: I did a quick visual check of the wiring, didn't spot anything obvious, but I'll take a closer look. I haven't noticed any coolant leaks, and the level seems fine. Regarding the system reset, is that something I can do myself, or does it require specialized equipment?

Given your meticulous examination and the persistent nature of the symptoms, it is increasingly probable that the temperature sensor itself is indeed compromised. While a system reset may temporarily alleviate the issue, it is unlikely to resolve the underlying hardware fault. I recommend a visit to a certified technician for a definitive diagnosis and component replacement. Continuing to operate the vehicle with a malfunctioning sensor may precipitate further complications within the HVAC system. Proceed with caution.

Massive thanks for the comprehensive response. Took it to my local garage, and they confirmed your suspicions. Turns out the temperature sensor for the heat exchanger had indeed given up the ghost. The part plus labour came to 980€, a bit steep, but at least the AC is blowing cold again. Appreciate the help!