Your boiler has detected an electrical fault with the sensor that monitors your hot water tank temperature, preventing it from heating your water correctly.

Tank NTC short circuit

What causes this fault?

This fault occurs when the electrical signal from the hot water cylinder's temperature sensor reaches an impossibly high level, usually because the internal wiring has failed or touched together. The most common reason is that the sensor has developed a 'short circuit' due to age or moisture, or the delicate wires connecting it to the boiler have been pinched or damaged.