Began inspection at the outdoor unit, tested contactor, and high voltage fuses. Proceeded to the furnace, tested for low-voltage power coming out of the transformer. Proceeding to the thermostat, I noticed that this was not a communicating thermostat while the rest of the system was all communicating. I also noticed that there was no common wire coming out of the thermostat. I only had red going to R, yellow going to Y, White going to W, and green going to G. These were the only wires accessible at the thermostat. I’ve then tested continuity on the green wire to ensure it was the same wire going to the furnace as the only wires hooked up to the furnace were blue yellow, white, and red. I advised the customer that the green wire needed to be hooked up into the furnace board and the thermostat replaced with the old communicating thermostat.