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

DIY fix possible

Turn the boiler off at its power switch, wait 30 seconds, then switch it back on to see if the code clears.Locate the outside temperature sensor, usually a small white or grey box on a shaded north or north-facing wall, and check that its cable is not cut, pinched, or loose at either end.Check that the cable has not been damaged by weather, pests, or garden work, and tighten any loose connections at the sensor end only.If the code returns after a reset, leave it alone and book a Gas Safe registered engineer to check or replace the sensor.