Why is 'Cool On' or 'Heat On' flashing on my T3 Pro display?
The compressor protection feature is engaged, which prevents the compressor from restarting too quickly after a shutdown. You simply need to wait 5 minutes for the system to restart safely without damage to the compressor. No action is required on your part. (Page 10)
From the official Honeywell T3 Pro Programmable Thermostat manual, page 10
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