Smart thermostat shows F.18 and the Vaillant app cannot access system data even though the device is online.

Safety: No electrical work required.

• 1. Check Vaillant service status: Visit Vaillant’s support site or notifications in the app to see if there is a known cloud outage.
• 2. Update app: Install the latest version of the Vaillant app from the official app store.
• 3. Re‑link account: In the app, remove the existing installation if present, then follow the guided process to add the device again using the QR code or serial number.
• 4. Verify region settings: Ensure the thermostat and app are set to the correct country/region supported by Vaillant’s cloud.
• 5. Reboot gateway: Power cycle the Vaillant internet module and wait a few minutes before trying again.
• 6. Contact Vaillant support: If F.18 persists, Vaillant may need to reset the device registration on their servers.