My heating or cooling system is not responding — what should I check?
First, verify the correct mode is set by pressing the Mode button, and confirm the desired temperature is set higher than room temperature for heat or lower for cool. Also check that the circuit breaker is on, the power switch for the system is on, and the furnace door is securely closed. If all checks pass, wait 5 minutes for the system to respond. (Page 10)
From the official Honeywell T3 Pro Programmable Thermostat manual, page 10
Have a follow-up question?
Answers from the official manual.
Common questions