Your boiler has detected very cold temperatures and has automatically turned itself on to prevent the internal pipes from freezing.

Anti-frost mode is active

What causes this fault?

The boiler's internal temperature sensors have detected that the water inside the unit has dropped below a safe threshold, typically around 5°C. To protect itself from damage caused by ice expansion, the boiler automatically fires up to circulate warm water until it reaches a safer temperature. This is a built-in safety feature rather than a mechanical failure.