Your boiler is struggling to work out how hard it needs to fire because the sensor that monitors the weather outside has stopped sending a signal.

Outside temperature sensor defective – check the connecting lead

What causes this fault?

This fault occurs when the boiler loses communication with the sensor mounted on your external wall, which tells the system how cold it is outside. The most common reasons are the sensor's internal components failing due to water ingress or the thin communication wire being damaged or corroded over time.