Your boiler has lost communication with the internal sensor that monitors the temperature of the water leaving the unit, usually due to a loose wire or a faulty part.

Flow sensor open circuit – check connections to flow sensor are in place

What causes this fault?

This fault occurs when the boiler's main control board loses the electrical signal from the sensor that monitors the temperature of the water being sent to your radiators. The most common reason is a physical failure of the internal thermistor or a loose connection in the wiring that prevents the system from safely tracking heat levels. Because the boiler no longer knows how hot the water is, it shuts down as a safety precaution to prevent overheating.