Your boiler is struggling to detect the temperature of the water coming into the unit, so it has shut down to prevent any damage.

Open circuit on inlet sensor

What causes this fault?

This fault occurs when the internal control board loses connection with the sensor that monitors the temperature of the water entering the boiler. The most common cause is a failure within the sensor's internal electrical components or a loose connection in the wiring that prevents the boiler from receiving a signal. Because the boiler no longer knows the temperature of the incoming water, it shuts down as a safety precaution to prevent overheating.