Your boiler is struggling to 'talk' to its control system, which prevents it from firing up correctly.

Simple cascade communication error.

What causes this fault?

This fault occurs when the boiler's control board receives conflicting information from the sensors that monitor the water temperature entering and leaving the unit. It typically happens because one of the sensors has failed internally or because a loose connection in the wiring is distorting the electrical signal. To protect the system from overheating or operating inefficiently, the boiler shuts down until the sensors provide consistent readings.