Vaillant smart thermostat shows F.09 and schedules cannot be saved or recalled correctly.

Safety: No high‑voltage work is required, but avoid opening the thermostat casing beyond user‑accessible parts.

• 1. Soft reset: Use the thermostat menu to perform a soft reset or restart. This often clears temporary memory errors.
• 2. Check for firmware updates: Via the Vaillant app or web portal, check if a firmware update is available. Apply updates with stable power and internet to avoid corruption.
• 3. Factory reset: Back up your schedules/settings if possible, then perform a factory reset from the thermostat menu. Reconfigure the system following the installation guide.
• 4. Avoid power cycling during writes: When saving schedules or updating firmware, do not switch off power or remove batteries.
• 5. Replace thermostat: If F.09 reappears after a full reset and reconfiguration, the internal memory may be defective. Replace the thermostat with an identical Vaillant model.