Your boiler has detected that the temperature sensors for the water going in and out are reporting incorrect or swapped readings, which has caused the system to shut down for safety.

NEGATIVE DIFFERENTIAL FLOW OR RETURN THERMISTOR FAULT

What causes this fault?

This fault occurs when the boiler detects that the water pipe leaving the unit is cooler than the pipe returning to it, which is physically impossible during normal operation. The most common reason is that the internal temperature sensors have failed or their wiring has become cross-connected, leading the boiler to shut down to prevent overheating. It can also be triggered by a complete lack of water circulation caused by a seized pump or a significant blockage in the system.