Your boiler has detected that a temperature sensor is not communicating correctly, which has caused the system to stop working to prevent overheating.

Insecure wiring to Flow Thermistor (upper LHS of heat exchanger), incorrect resistance values (9.7-10.3 kΩ at 25°C), or insecure PCB connections.

What causes this fault?

This fault occurs when the boiler's internal computer loses its ability to accurately monitor the temperature of the water leaving the heat exchanger. The most common reason is that the thermistor sensor has physically degraded or developed an electrical failure, but it can also be caused by a loose connection or a fault within the main control board. Because the boiler can no longer ensure the water isn't getting dangerously hot, it shuts down as a safety precaution.