Your boiler has lost its connection to a separate add-on control unit, so the extra heating controls it manages may not be working, though the boiler itself may still run.

DIY fix possible

Turn off the boiler at its on/off switch and at the fused spur or isolator, wait 5 minutes, then switch it back on to reset the communication link.With the power off, check that the cable between the boiler and the EA1 extension box is firmly plugged in at both ends, and look for any obvious damage to the cable. Do not open the boiler casing or the EA1 box.If the C1 code returns after the reset, or the cable is damaged, stop and book a Viessmann-registered heating engineer.