Your boiler has detected a fault with the temperature sensor that controls your hot water, meaning your water may not reach the correct temperature or might not heat up at all.

Fault codes 6x appear if a problem exists with the secondary (DHW) thermistor.

What causes this fault?

This fault occurs when the internal sensor responsible for monitoring hot water temperature starts sending incorrect readings or stops communicating with the main control board. The most common reason is that the sensor has suffered from electrical failure or internal corrosion over time, preventing the boiler from knowing how hot the water actually is. As a safety precaution, the boiler stops heating the water to prevent it from becoming dangerously hot.