Your boiler is struggling to accurately measure the water temperature, causing it to shut down to prevent overheating.

Flow and/or Return thermistor fault

What causes this fault?

This fault occurs when the boiler's control board receives conflicting or incorrect temperature signals from the sensors on the main pipes. It usually happens because these sensors have drifted out of their correct calibration or the wiring connecting them has degraded. As a safety precaution, the boiler shuts down because it can no longer accurately monitor if the water is getting too hot.