Vaillant smart thermostat shows F.03 and cannot connect to the Vaillant internet module; app control unavailable.

Safety: While this is low‑voltage communication, always unplug or power down the boiler and gateway before unplugging internal connectors.

• 1. Identify the internet module: Locate the Vaillant internet module (e.g. VR 920, VR 921, sensoNET) usually near the boiler or in the wiring center. Confirm its LEDs are lit and not indicating a fault (refer to its manual).
• 2. Check low‑voltage wiring: Inspect the data cable between the thermostat and the internet module. Ensure plugs are fully seated and no pins are bent. If it uses eBUS, verify correct terminals and no shorts.
• 3. Power cycle: Turn off power to the boiler and gateway for 2 minutes. Turn on the boiler first, then the gateway, then the thermostat. Wait several minutes for them to re‑establish communication.
• 4. Re‑pair the devices: On the thermostat menu, navigate to connectivity or gateway settings and follow the pairing procedure described in the Vaillant manual. On some models you must press a pairing button on the VR 920/921.
• 5. Check firmware/app: Update the Vaillant app and check if a firmware update is available for the thermostat or gateway. Some communication errors are resolved by firmware updates.
• 6. Replace suspect module: If F.03 remains and wiring is sound, the internet module may be defective. Replacement should be done by a Vaillant‑approved installer to ensure correct configuration.