Your boiler has lost its ability to accurately measure the temperature of your hot water, which may cause it to run poorly or stop providing hot water altogether.

A problem exists with the secondary (DHW) thermistor.

What causes this fault?

This fault occurs when the domestic hot water sensor becomes damaged or begins sending an incorrect electrical signal to the boiler's control board. The most common reason is internal wear and tear or minor water leaks causing corrosion on the sensor's electrical pins, preventing the boiler from knowing how hot your water is. Because the boiler cannot safely monitor the temperature, it will stop firing for hot water to prevent overheating.