What do I do if the heating or cooling system doesn't respond?
Press the Mode button to set the correct system mode (Heat or Cool), and ensure the desired temperature is set higher than inside temperature for heat or lower for cooling. Check the circuit breaker and reset if necessary, verify the power switch is on, ensure the furnace door is closed securely, and wait 5 minutes for the system to respond. (Page 8)
From the official Honeywell T4 Pro Thermostat manual, page 8
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