Your boiler has lost its connection to the room thermostat or controller, so it may not be heating your home according to your settings, although it is likely still safe to run.

DIY fix possible

Switch the boiler off at its on/off switch and at the mains supply, then check the room thermostat or controller has working batteries or power and that its display is lit.With the boiler off, check the two-wire connection between the room thermostat and the boiler's control terminals is firmly in place and not loose, damaged or disconnected. Do not touch any gas components or open the boiler casing beyond the terminal cover.Restore power, wait a few minutes, then reset the boiler using its reset button or the reset option on the controller. If the code clears and heating responds to your settings, no further action is needed. If the code returns, book a Gas Safe registered engineer.