Your boiler is struggling to accurately measure the temperature of your hot water, which usually prevents it from heating the water correctly.

DHW NTC sensor defective (short or open circuit)

What causes this fault?

This fault occurs when the sensor responsible for monitoring your hot water temperature fails to send a clear signal back to the main control board. The most common reason is that the sensor has developed a physical breakdown or electrical short after years of constant temperature changes, though loose or corroded wiring can also mimic this failure. Because the boiler no longer knows how hot the water is, it stops heating it as a safety precaution to prevent scalding.