Your boiler has stopped working because it has detected an electrical fault with the internal sensor that monitors the temperature of the water leaving the unit.

Short circuit – flow–NTC (< 130 °C), NTC– plug shorted to casing, NTC defective

What causes this fault?

This fault occurs when the electrical signal from the sensor that monitors the outgoing water temperature is interrupted, usually due to a physical failure inside the component. The most common reason is that moisture or heat stress over time has damaged the sensor's internal wiring, causing it to send an incorrect 'short-circuit' signal to the main control board. If the sensor cannot safely report the temperature, the boiler will shut down as a safety precaution.