My heating or cooling system is not responding — what should I do?
First, press the SYSTEM button to set the correct mode and verify the temperature is set higher than the indoor temperature for heat, or lower for cool. Also check that the circuit breaker is not tripped, the power switch is on, and the furnace door is securely closed. If none of these resolve the issue, wait 5 minutes for the system to respond. (Page 12)
From the official Honeywell TH5110D Home Programmable Thermostat manual, page 12
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