Display shows 'Cool On' flashing and AC does not start

Safety: Do not repeatedly cycle the thermostat on and off; this can damage the compressor.

• Wait the delay period: Most Honeywell Home smart thermostats enforce a 3–5 minute delay after the compressor turns off. Leave the thermostat in Cool mode with a low setpoint and wait at least 5 minutes.
• Verify system switch: Ensure the thermostat is set to Cool and the fan is set appropriately (Auto or On).
• Check for error at equipment: If 'Cool On' continues flashing for more than 10 minutes, check the furnace/air handler for tripped float switch, blown low‑voltage fuse, or error lights.
• Confirm thermostat settings: In advanced installer settings, verify that compressor protection time is not set excessively high.
• Call technician if no cooling: If the delay clears but the outdoor unit never starts, there may be a problem with the contactor, control board, or low‑voltage wiring.