Why won't my Honeywell Pro Series thermostat start the AC or heat?

The thermostat may not activate AC or heat if the HVAC system is not responding, the thermostat is not properly wired, or the system is not powered on. If the thermostat is on hold and set to a temperature that wouldn't trigger heating or cooling, the system may appear inactive. Check that the circuit breaker is on, the furnace door is closed, and the power switch is enabled.