Your boiler has lost contact with its outdoor thermometer, so it is running at a pre-set steady temperature to keep your home warm while it ignores external weather changes.

Outside temp. sensor faulty - heating back-up operation active

What causes this fault?

This fault occurs when the boiler's control board can no longer detect a signal from the sensor located on the outside of your home. The most common reasons are that the external sensor has developed an electrical fault due to moisture ingress, or the thin wiring connecting it to the boiler has become damaged or disconnected.