Why does my T4 Pro display 'Cool On' or 'Heat On' flashing?
The flashing message indicates that compressor protection is engaged. Damage can occur if the compressor restarts too soon after shutdown, so the system waits a few minutes for safe restart. Wait approximately 5 minutes for the system to restart safely without damaging the compressor.
From the official Honeywell T4 Pro Thermostat manual, page 5
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