Your boiler has detected that a temperature reading from its internal water system is not what it expects, meaning it has safely shut down to prevent damage.

Flow sensor out of range. Sensor measures a temperature outside normal working range.

What causes this fault?

This fault occurs when the boiler's internal computer receives a temperature reading that is physically impossible or outside of its safety limits. It is usually caused by a chemical failure inside the flow sensor itself or a loose connection in the wiring that interrupts the electrical signal. Because the boiler can no longer accurately monitor how hot the water is, it shuts down to prevent the system from overheating.