Thermostat shows A5 and cannot connect to the Viessmann cloud; app control is unavailable.

Safety first: This is a low‑voltage communication issue; no mains work is required. Do not open the thermostat beyond the user‑accessible cover.

• Check internet access: Verify that other devices on the same network can access the internet. Restart the router if necessary.
• Check Wi‑Fi signal: Ensure the thermostat or Vitoconnect gateway is within good range of the router. If signal is weak, move the router closer or use a Wi‑Fi extender.
• Re‑enter Wi‑Fi credentials: Using the Viessmann app, go to device settings and reconfigure the Wi‑Fi connection, ensuring the correct SSID and password are entered.
• Check Viessmann service status: If possible, check Viessmann’s service or support page for any reported cloud outages.
• Reboot devices: Power cycle the router, then the thermostat or Vitoconnect gateway. If A5 persists, contact Viessmann support to verify account and server status.