The U0164 OBDII fault code basically means that the HVAC Control Module (Ventilation, Heating, and Air Conditioning) and other vehicle modules have been unable to communicate with each other. The circuit used for such communication is the Control Area Network or CAN bus.
The HVAC control module is normally found in the instrument panel and receives inputs from different sensors, which are often connected directly to it. With such input signals, the module can control the interior temperature of the vehicle.