Your boiler has stopped working because it thinks the internal heat sensor is damaged or has an electrical fault, preventing it from monitoring temperature safely.

Heat exchanger temp sensor short circuit

What causes this fault?

This fault occurs when the electrical signal from the heat exchanger's temperature sensor reaches an impossible level, usually because the sensor has failed internally. The most common reason is that the sensor has developed a short circuit, meaning it can no longer accurately tell the boiler how hot the water is, causing the system to shut down for safety.