Your boiler has detected a problem with the temperature sensor that controls your hot water, meaning you likely have no hot water or the temperature is inconsistent.

DHW thermistor fault (see 7.2.5.2)

What causes this fault?

This fault occurs when the sensor responsible for measuring the temperature of your hot water fails or sends an incorrect signal to the main control board. The most common reason is that the internal components of the sensor have worn out over time due to constant heating and cooling, or the electrical connection has become loose or corroded. Because the boiler no longer knows how hot the water is, it shuts down the hot water function as a safety precaution.