Your boiler has become too hot and has shut itself down as a safety precaution to prevent damage.

Overheat (With V9 Primary Control PCB, will display 3 horizontal lines upon reset and not refire until temperature drops below 50oC)

What causes this fault?

This fault occurs when the water inside the boiler exceeds a safe temperature, triggering a safety sensor to cut the power. The most common reasons are a pump failure that stops water from moving or a blockage within the main heat exchanger that prevents heat from transferring correctly. It can also be caused by air trapped in the system or a faulty sensor providing an incorrect reading.