There are many common reasons why a Honeywell thermostat is not turning your AC on, and most are not related to the thermostat itself. First, check the most basic settings: make sure the thermostat is set to Cool mode and that the setpoint is at least 3 degrees lower than the current room temperature. If it is in Auto or Off, it will not call for cooling. Second, check for power issues. If the thermostat is battery powered, replace the batteries. If it is a C-wire powered smart thermostat, check the circuit breaker for the indoor air handler and the outdoor condenser unit, and check the furnace power switch. A tripped breaker or blown 3-amp fuse on the furnace control board will prevent the AC from starting. Third, check for protection delays. Many Honeywell thermostats display Waiting, Cool On, or a flashing snowflake icon for up to 5 minutes to protect the compressor. Fourth, inspect your air filter and condensate drain. A clogged filter can cause the system to overheat and shut down, and a full condensate drain pan with a tripped float switch will cut power to the cooling. Finally, check the wiring behind the thermostat for loose Y, R, and G wires, which control cooling.
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