Your boiler's burner stays lit even when it should turn off, meaning it isn't responding to your thermostat or timer settings.

The burner will remain alight until switched off by the control thermostat, any remote control or the programmer.

What causes this fault?

This fault occurs when the boiler loses its ability to regulate the flame, causing the burner to stay lit regardless of your heating settings. The most common reason is a malfunctioning main circuit board or a faulty gas valve that has become stuck in the open position. It can also be caused by a wiring error in your external thermostat or programmer that is continuously sending a 'heat on' signal.