Your boiler's hot water sensor has failed or has a broken connection, meaning you likely won't have any hot water.

The hot water negative thermistor coefficient (NTC) sensor is defective – the hot water sensor or connecting leads need checking.

What causes this fault?

This fault occurs when the boiler's internal computer loses communication with the sensor responsible for monitoring hot water temperature. The most common reason is that the sensor has physically failed due to age or internal wear, though it can also be triggered by loose or corroded electrical wiring connecting the sensor to the main control board.