What should I do if my Honeywell T4 thermostat display is blank?
If the display is blank, check and reset the circuit breaker 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 Thermostat manual, page 8
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