Your boiler has stopped working because it can no longer accurately measure the temperature of the water inside the system.

Burner blocked. Lead break - boiler water temperature sensor.

What causes this fault?

This fault occurs when the boiler's internal computer loses communication with the sensor that monitors the main water temperature. This is typically caused by the sensor itself failing due to age or the electrical wiring harness becoming damaged or loose. Because the boiler no longer knows how hot the water is, it shuts down the burner as a safety precaution to prevent overheating.