The boiler's outdoor weather sensor is sending a faulty signal, so it can't adjust heating to the weather, but it will usually keep heating and hot water working normally.

DIY fix possible

Turn the boiler off at the on/off switch, wait 5 minutes, then turn it back on and check whether the fault clears.Find the outdoor temperature sensor, a small white or grey box on an outside wall, usually on the north or shaded side and often under a cover. Check that its cable is not pinched, cut, or chewed, and that the sensor is not damaged or full of water.If the cable is damaged, do not repair it yourself. Contact a Gas Safe registered engineer, as the boiler connection and sensor replacement should be done by a qualified person.If the fault returns after a reset, book a Gas Safe engineer to test or replace the outdoor sensor.