Your boiler has detected that its temperature sensors are giving conflicting readings, which has caused it to stop working for safety.

Permanent drift between flow sensor and return sensor. Check central heating flow and return thermistor connection. Thermistor defective

What causes this fault?

This fault occurs when the sensors monitoring the water leaving and returning to your boiler disagree on the temperature by a significant margin. The most common reason is that one of these sensors has become faulty or 'drifted' over time, sending incorrect data to the internal computer. In some cases, it can also be caused by a circulation issue or a failing control board misinterpreting the signals.