The boiler has noticed a problem with the sensor that measures the outdoor temperature, so it is running at a fixed level instead of adjusting to the weather, but it should still heat your home and water.

DIY fix possible

Turn off the boiler at its on/off switch and, if you can, at the fused spur or isolator supplying it. Do not open the boiler casing.Find the outside temperature sensor, usually a small white or grey box on a north or shaded wall, and check it for visible damage, cracks, or water inside the housing.Follow the sensor cable from the sensor as far as you can see. Look for cuts, crushed or pinched sections, or cable that has been stapled too tightly, and check that the connections at the sensor are secure and dry.Turn the power back on and press the reset button on the boiler, or switch the boiler off and on again. If the code returns, or you found damage, contact a Gas Safe registered engineer to test or replace the sensor and cable.