Your boiler has detected an electrical fault with one of its main internal temperature sensors, causing the system to shut down for safety.

Safety sensor short-circuited.

What causes this fault?

This fault occurs when the boiler's safety sensor, which monitors for overheating, sends an abnormal electrical signal to the control board. The most common reason is that the sensor's internal wiring has failed or a moisture leak has caused a short circuit. Because this sensor is responsible for preventing the boiler from becoming dangerously hot, the system will not start until the part is replaced.