What should I do if the Infinity Thermostat display says 'Indoor Unit Not Found'?

First, recheck the ABCD wiring on all devices to ensure all colors match for every terminal. Press the left-side button to try again. If the display still reads 'Indoor Unit Not Found,' disconnect accessories and all devices from the ABCD connection and connect the User Interface directly to the indoor unit with a short piece of thermostat wire. Add other devices one at a time to determine where the communication issue exists.