Your boiler's remote monitoring module isn't talking to Viessmann's system, which usually means a connection or signal problem rather than a fault with the heating itself.

DIY fix possible

Check that the Vitocom 100 module has power, using the indicator light on the unit. If it is off, check the plug and socket and make sure it is switched on.Make sure the module's GSM aerial cable is firmly connected and that the aerial is not hidden behind metal or in a cupboard where the mobile signal is weak.Confirm the SIM card is active with your provider and has data available. A SIM that has run out of credit or data can cause this error.Switch the boiler off at its isolator or power switch, wait five minutes, then switch it back on. Check whether the Cd code clears.If the code returns, contact a Viessmann-registered engineer. Do not open the boiler casing or the gas side yourself.