The T-Stat was wired correctly, after going through 3 different c4 t-stats... One never worked with AC, one had a faulty LCD and the 3rd arrived DOA during install... The issues are the configuration options and how it runs the system. It was functioning properly, just not running the system efficiently. I had done a ton of complex programming to attempt to overcome its short comings, like running the fan for 30 minutes after the AC shut off, etc. However, Nest just does this stuff right out of the box, and also factors outside temp and humidity as well as indoor humidity to determine the best schedule and operation modes. There is no easy way to integrate or program off humidity with a C4 t-stat (yes, 3rd party options do exist, but a Nest does it automatically). And lastly, the appearance of the C4 T-stat looks like it escaped the 80s plastic rectangle invasion... So, after the 3rd T-stat from C4, it was functioning properly most of the time. Occasionally it would sync with the system and still be running its own programming or reset and set the heat to 88F but other than those quirks, it was find. But, ultimately, I'm loving my Nest even without the integration with C4. Love the energy reports each day to see how the system is performing.