Your boiler is receiving a signal to provide heat or hot water, which typically means it is operating as normal.

DEMAND ON

What causes this fault?

This is not actually a fault, but a status indicator showing that your thermostat or programmer has successfully requested heat. It confirms that the internal control board has received the electrical signal to start the ignition process for your heating or hot water. If the boiler is displaying this but not firing up, the issue likely lies with a secondary component rather than the initial request for heat.