Your boiler has likely overheated or has an issue with its internal fan system, causing it to shut down for safety.

High limit thermostat fault or air flow switch fault

What causes this fault?

This fault occurs when the boiler's internal components detect that the unit is either getting too hot or that the fan isn't safely clearing waste gases. The most common reasons are a blockage in the flue, a failing circulation pump, or a simple mechanical failure of the safety sensors themselves.