Your boiler has stopped working because one of its internal temperature sensors has developed an electrical fault and is giving an incorrect reading.

Short circuit – return–NTC (< 130 °C) NTC defective | Cause: NTC– plug shorted to casing

What causes this fault?

This fault occurs when the sensor responsible for monitoring the temperature of the water returning to the boiler fails or develops an electrical short. The most common reason is that the sensor's internal insulation has broken down or the connecting wires have touched the metal casing, sending an incorrect electrical signal to the main control board.