When I arrived, I turned the AC on at the thermostat however the AC did not kick on to run. I connected to the system through an app on my phone and found that the AC unit was not communicating with the furnace. I then went outside and saw an error code on the AC. The error code stated that the AC was low on refrigerant. I went ahead and restarted the system, and then the AC came on and was running, so I checked refrigerant pressures. The temperature outside is too cold to properly check refrigerant however I am able to get a good idea on refrigerant levels. Refrigerant pressure check shows that the unit should be OK on refrigerant. The communication error is normally the control board on the outside unit. Since the furnace works perfectly fine and is connecting properly through the app. I diagnosed that the outside unit control board needs replaced. I talked with customer about options with that. Customer would like to talk with spouse before making any decisions.