What should I do if the display is blank on my T4 Pro thermostat?
If the display is blank, check the circuit breaker and reset if necessary, ensure the power switch for your heating and cooling system is on, verify the furnace door is closed securely, and make sure fresh AA alkaline batteries are correctly installed. (Page 8)
From the official Honeywell T4 Pro Thermostat manual, page 8
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