The thermostat backlight is very dim or flickers intermittently.

Steps:

• Check supply voltage: Measure between R and C at the thermostat; ensure ~24VAC is present and stable.
• Reseat thermostat: Remove and reattach the thermostat to its sub-base to ensure good pin contact.
• Inspect for damage: Look for signs of moisture or corrosion around the display edges.
• If voltage is good but the backlight remains dim/flickers: The backlight circuit or LEDs are failing; replace the thermostat.