Your boiler has detected a problem with the sensor that monitors the temperature of the water leaving the unit, usually resulting in the system shutting down for safety.

Flow thermistor error

What causes this fault?

This fault occurs when the control board loses communication with the sensor that monitors the temperature of the water as it leaves the boiler. The most common reason is that the sensor has physically failed or 'drifted' out of its correct electrical range, but it can also be caused by loose or damaged wiring. Because the boiler no longer knows how hot the water is, it shuts down as a safety precaution to prevent overheating.