Do I need to reprogram the thermostat if I change the batteries?
If you insert new batteries within 20 to 30 seconds of removing the old ones, you do not have to reprogram the thermostat. However, if the display is blank, the batteries are dead or incorrectly installed and you must reprogram according to pages 14 and 15.
From the official Honeywell CT3355 Home Programmable Thermostat manual, page 8
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