Your boiler has detected an electrical fault with the sensor that monitors water returning from your radiators, causing the system to shut down for safety.

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

What causes this fault?

This fault occurs when the electrical sensor that monitors water returning to the boiler develops an internal short-circuit. It is often caused by heat degradation over time or water damage to the sensor's wiring, which leads to the control board receiving an incorrect electrical signal. Because the boiler can no longer accurately monitor temperature, it shuts down as a safety precaution.