Your boiler has detected an issue with the sensor that monitors the temperature of the water leaving the unit, meaning it cannot safely regulate heat.

Flow Thermistor Fault

What causes this fault?

This fault occurs when the boiler's control board loses communication with the sensor that monitors the temperature of the water being sent to your taps and radiators. The most common reason is that the sensor has physically failed or developed an electrical short, though loose or damaged internal wiring can also prevent the boiler from reading the temperature correctly.