Your boiler has lost its connection to the room thermostat or its control unit, so it may not be responding to heating requests correctly, but this is usually fixable and is not a safety risk.

DIY fix possible

Turn off the boiler at its isolator switch or fused spur, wait about 30 seconds, then turn it back on to reset the connection.Check that the room thermostat or controller has working batteries, and replace them if needed.Look at the wiring between the thermostat and the boiler for loose or damaged cables, but do not open the boiler casing.If the code returns after the reset, contact a Gas Safe registered engineer to check the OpenTherm connection and controller.