Your boiler has lost its connection to Viessmann's online service, which is usually a temporary communication problem and does not normally stop your heating or hot water from working.

DIY fix possible

Check that your home internet router is switched on and that other devices in the house can get online.Turn the boiler off at its on/off switch, wait about 30 seconds, then switch it back on.Wait 15 to 30 minutes and see whether the E12 code clears on its own, as the connection can be re-established later.If the code returns, check that the boiler's internet module (Vitoconnect) is firmly connected and that your Wi-Fi signal reaches the boiler's location.If the code persists after these steps, contact a Gas Safe registered engineer or Viessmann support to check the internet module and network settings.