Your boiler has lost contact with the room thermostat or room temperature sensor, so it may not be controlling the heating by room temperature as normal, though it can often still heat your home.

DIY fix possible

Check the wireless room thermostat or sensor (often a Vitotrol unit) for low batteries and replace them with fresh ones if needed.Make sure the room sensor is within range of the boiler, is not blocked by metal objects, and is not in direct sunlight, draughts, or near a radiator.Turn the boiler off at its on/off switch, wait 30 seconds, then turn it back on and see whether the code clears.If the code returns, contact a Gas Safe registered engineer to check the sensor wiring and replace the room sensor if faulty. Do not open the boiler casing yourself.