Why is 'Cool On' or 'Heat On' flashing on my thermostat?
The flashing message indicates that the compressor protection feature is engaged. This prevents damage to the compressor by forcing it to wait several minutes before restarting safely after shutdown. Wait 5 minutes for the system to restart without damage. (Page 5)
From the official Honeywell T4 Pro Thermostat manual, page 5
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