Your boiler has stopped working because an internal temperature sensor has developed an electrical fault and can no longer read the heat levels correctly.

Supply sensor short-circuited.

What causes this fault?

This fault occurs when the electrical resistance in the main temperature sensor drops to an abnormal level, suggesting a short circuit. The most common reason is that the sensor's internal components have perished over time, or moisture has compromised the electrical connections. Because the boiler can no longer safely monitor how hot the water is getting, it shuts down as a safety precaution.