Your boiler has lost its connection to an external communication module, which usually affects remote monitoring or controls rather than the heating and hot water themselves, so it may still be working normally.

DIY fix possible

Turn the boiler off at its isolator switch or fused spur and wait about 60 seconds, then switch it back on.Check that the communication cable running from the boiler to the Vitocom 100 unit is firmly plugged in at both ends, without touching any wiring inside the boiler casing.If the code clears and the boiler runs normally, keep an eye on it. If Cd returns, book a Gas Safe registered engineer to check the Vitocom 100 module and the KM BUS wiring.