Why is my thermostat display blank?
A blank display can be caused by several issues. Check your circuit breaker and reset it if necessary, ensure the power switch at the heating and cooling system is on, make sure the furnace door is closed securely, and verify that fresh AA alkaline batteries are correctly installed. (Page 10)
From the official Honeywell TH3000 Programmable Thermostat manual, page 10
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