Thermostat screen is very dim or unreadable, but heating still responds.

Safety first: No mains exposure if working only on the thermostat.

• Adjust contrast/brightness: In the display settings, increase brightness or contrast if available.
• Replace batteries: Low battery voltage can cause dim displays; fit new batteries.
• Check ambient light sensor: Some models dim automatically; ensure the sensor is not covered.
• If still dim: The LCD/backlight is failing. The thermostat usually needs full replacement, as the display is not a separate spare.