Your boiler has detected that a temperature sensor is disconnected or has failed, which is currently preventing the heating from starting to protect the system from freezing.

Central Heating flow thermistor disconnected (frost protection).

What causes this fault?

This fault occurs when the boiler's control board loses communication with the temperature sensor on the main heating pipe. The most common reason is that the sensor has developed an internal electrical failure or the wiring connecting it to the brain of the boiler has become loose or damaged.