Thermostat backlight flickers or the display intermittently dims and brightens.

Safety first: For mains-powered units, isolate power before removing the thermostat from the wall.

Steps to check:

• Tighten terminals: Check that live and neutral wires are firmly clamped in the thermostat backplate.
• Check supply stability: Use a multimeter to verify that the supply voltage is stable and within specification.
• Replace batteries: On battery models, fit new batteries to rule out low voltage.
• Observe behaviour: If flicker occurs when pressing buttons or when the backlight turns on, the internal power circuit may be marginal.

Resolution: If flickering persists after ensuring good supply and connections, replace the thermostat head.