Your boiler has stopped receiving the outside temperature reading from its wireless sensor, so it is assuming it is freezing outside and may heat your home more than it needs to, though it should still keep working.

DIY fix possible

Check the batteries in the wireless outside temperature sensor and replace them with fresh ones if they are low or flat.Make sure the sensor is mounted on a shaded north or north-facing wall, is not covered by plants or debris, and is not blocked by metal objects or thick walls between it and the boiler area.Press the reset button on the boiler or switch it off at the isolator for about 30 seconds, then switch it back on and see whether code 19 clears. If it returns, contact a Gas Safe registered engineer.