Vaillant smart thermostat shows F.15 and the display is very dim or unreadable, sometimes flickering.

Safety: Isolate power to the thermostat circuit before removing it from the wall plate.

• 1. Check supply voltage: Measure the low‑voltage supply at the thermostat terminals (if accessible) and confirm it matches Vaillant specifications. If low, investigate the boiler or PSU.
• 2. Reseat thermostat: Remove the thermostat from its wall plate and re‑seat it to ensure good contact on the connector pins.
• 3. Inspect for damage: Look for cracks in the display, signs of impact, or moisture inside the housing.
• 4. Test in another location: If possible, connect the thermostat to a known‑good Vaillant boiler or test rig to rule out supply issues.
• 5. Replace thermostat: Display and backlight components are not usually serviceable; replace the thermostat if F.15 persists and the display remains faulty.