Your boiler is confused because the temperature sensors measuring water going out and coming back are giving contradictory or swapped readings.

Fault: NTC temp. diff. implausible Temperature difference is implausible. Check: Flow/return temperature sensor (correct thermal connection, sensors are inverted).

What causes this fault?

This fault occurs when the boiler's internal computer notices that the temperature sensors for the water leaving and returning to the unit are giving conflicting or 'impossible' data. The most common reason is that the sensors have been clipped onto the wrong pipes during a repair, or a sensor has become loose and is no longer reading the temperature correctly.