Your boiler's internal temperature sensors have developed a fault, meaning the system cannot safely monitor the water temperature and has shut down as a precaution.

Internal PCB fault

What causes this fault?

This fault occurs when the boiler's internal temperature sensors stop providing a reliable reading to the control board. The most common reason is that the thermistor has physically degraded or developed an electrical short, though it can also be caused by loose wiring connections between the sensor and the main computer. Because the boiler no longer knows how hot the water is, it shuts down to prevent the unit from overheating.