Your boiler has lost its ability to accurately measure the water temperature inside the system, which is preventing it from heating up safely.

A problem exists with the primary thermistor.

What causes this fault?

This fault occurs when the internal sensor responsible for monitoring water temperature fails or loses its connection to the main control board. The most common reason is that the sensor has simply worn out over time or the electrical pins have become corroded, preventing the boiler from knowing if the water is getting too hot. Because the boiler can no longer monitor safety temperatures, it shuts down to prevent any internal damage.