What does the 'Cool On' or 'Heat On' flashing message mean?
The flashing 'Cool On' or 'Heat On' message indicates that the compressor protection feature is engaged. You should wait 5 minutes for the system to restart safely without damaging the compressor, which is the purpose of this built-in protection feature.
From the official Honeywell T6220D Programmable Thermostat manual, page 15
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