Your boiler has lost track of the outside temperature reading, so it can't adjust its heating output to the weather, but it will usually keep heating and supplying hot water as normal.

DIY fix possible

Turn the boiler off at its power switch, then find the outside temperature sensor, which is usually a small white or grey box on an external north-facing wall.Check the sensor cable for visible damage, chewed wires, or water getting in, and make sure the plug or terminal connections at the sensor are firm and dry.Turn the boiler back on and press the reset button on the boiler panel. If code 14 returns, leave the boiler running and book a qualified heating engineer to test or replace the outside sensor.