Your boiler is not turning off when the water gets hot enough, which runs the risk of the system overheating.

Main burner fails to shut down when water reaches pre-determined temperature - Thermostat out of calibration or faulty.

What causes this fault?

This fault occurs when the internal sensors fail to tell the boiler to stop firing once the water has reached its target temperature. The most common reason is that the control thermostat has lost its calibration or become stuck in the 'on' position, meaning the burner stays lit indefinitely. This creates a safety risk as the water inside the unit could potentially reach boiling point if the safety limits do not intervene.