Your boiler has detected that the temperature readings between two sensors don't match up, which prevents it from heating your water safely.

Flow return NTC error – temperature difference too great

What causes this fault?

This fault occurs when the boiler's control board detects that the temperature readings from the flow and return pipes don't match up logically, suggesting one of the sensors is faulty or out of calibration. Essentially, the boiler can no longer trust the data it is receiving about how hot the water is, so it shuts down as a safety precaution. While it is usually a sensor failure, it can occasionally be caused by a wiring issue between the sensors and the main control unit.