Your boiler is struggling to provide hot water because the internal thermometer responsible for measuring its temperature has developed an electrical fault.

What causes this fault?

This fault occurs when the electrical component that monitors your hot water temperature has failed, essentially sending an 'impossible' signal to the boiler's computer. The most common reason is that the sensor has either burnt out internally or suffered from a build-up of moisture that has caused a short circuit in the wiring. Because the boiler no longer knows how hot the water is, it prevents the system from running to avoid overheating or scalding.