Your boiler has detected an issue with the sensor that monitors the temperature of the water leaving the unit, causing it to shut down as a safety precaution.

Primary (flow) thermistor fault

What causes this fault?

This fault occurs when the boiler's internal computer receives an incorrect or missing signal from the sensor that tracks how hot the water is. The most common reason is that the sensor itself has worn out and failed, though it can also be caused by a loose connection or damaged wiring. Because the boiler no longer knows the exact temperature of the water, it shuts down to prevent the system from overheating.