Your boiler is having trouble communicating with an external control sensor, which means it cannot accurately measure the temperature of the hot water leaving the unit.

What causes this fault?

This fault occurs when the boiler loses its electronic connection to the sensor responsible for monitoring hot water temperature. The most common reason is a loose or damaged wiring connection between the main control board and the external sensor, or the sensor itself has developed a fault and stopped sending a signal. Because the boiler can no longer confirm how hot the water is, it stops heating as a safety precaution.