Your boiler has shut down because a safety sensor has detected that the main heating component is getting too hot or has a broken internal fuse.

Overheating protection on primary heat exchanger triggered | Cable or cable connection of fuse in the primary heat exchanger defective, or primary heat exchanger defective

What causes this fault?

This fault occurs when a dedicated safety fuse inside the main heat exchanger has melted or 'tripped' to prevent the boiler from melting down. It is triggered when the heat exchanger becomes dangerously hot, often due to a lack of water flow, a severe blockage, or a build-up of air inside the central heating system. Once this fuse has blown, it is a hardware failure that prevents the boiler from firing up for safety reasons.