The boiler's outside weather sensor appears to be faulty, so it is heating your home as though it were freezing cold outside, which can make it run more than needed but it is usually not dangerous.

DIY fix possible

Switch the boiler off at its on/off switch and at the fused spur, wait 2 minutes, then switch it back on and check whether the code clears.Locate the outside temperature sensor, usually a small white or grey box on a north or north-east facing external wall, and check it visually for physical damage, a loose or disconnected cable, or water ingress.If the cable and sensor look undamaged, leave everything else alone and book a Gas Safe registered engineer to test or replace the sensor. Do not open the boiler casing.