Your boiler has shut down because the internal gases leaving the system have become too hot, which is a safety feature to prevent damage to the internal components.

What causes this fault?

This fault occurs when the internal safety sensor detects that the exhaust gases have reached a dangerously high temperature, often due to a lack of water flow or a build-up of debris inside the heat exchanger. It essentially means the heat being produced isn't being carried away into your radiators efficiently enough, causing the internal components to overheat. In some cases, it can also be triggered by a faulty sensor providing an incorrect reading or damaged internal insulation.