How do I adjust the backlight brightness on my T4 Pro thermostat?
Press Menu, navigate to LITE using the arrow buttons, and press Select. The current brightness setting (1-5) will display. Use the arrow buttons to adjust brightness to your preference, then press Select to save and exit. Note that backlight is only available if the C (common) wire is connected.
From the official Honeywell T4 Pro Thermostat manual, page 5
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