The boiler has lost the signal from its outside weather sensor, so it can't adjust heating to the weather, but it can usually still heat your home and water.

DIY fix possible

Turn the boiler off at its power switch, wait 30 seconds, then turn it back on to clear the fault and see if it returns.Look at the outside temperature sensor on the exterior wall, usually a small white or grey box near a window or under the eaves, and check that its cable is not cut, pinched, or damaged and that the cover is not cracked or letting in water.If the cable is damaged, or the fault comes back after the reset, contact a Gas Safe registered engineer to test or replace the sensor. Do not open the boiler casing yourself.