Your boiler has stopped working because one of its internal sensors has developed an electrical fault and can no longer measure the temperature of the water leaving the unit.

Flow temperature sensor short circuit

What causes this fault?

This fault occurs when the electrical connection within the flow temperature sensor fails, causing it to send an incorrect signal to the main control board. The most common reason is that the sensor has developed an internal short circuit due to age, or moisture has compromised the wiring connections. Because the boiler can no longer safely monitor how hot the water is, it shuts down as a safety precaution.