Your boiler has lost clear communication with its external heating controller or thermostat, so it may not respond properly to your heating settings, though it can often still run.

DIY fix possible

Turn off the boiler at its on/off switch and at the mains supply, wait 2 minutes, then turn it back on to reset the communication.Check that the batteries in your room thermostat or controller are fresh and replace them if needed.Look at the wiring between the boiler and the room thermostat or controller, and make sure no wires are loose or damaged. Do not open the boiler casing or touch any internal gas components.If the code returns, leave the boiler on and contact a Gas Safe registered engineer to check the OpenTherm connection and controller.