Your boiler is currently preparing to fire up the burners to heat your water or radiators after a brief pause.

Ignition sequence (warm start)

What causes this fault?

This is a normal part of your boiler's operating cycle rather than a mechanical failure. It occurs when the boiler has briefly paused because it reached the desired temperature and is now re-engaging the ignition process to maintain the heat for your home. The 'S' prefix indicates a status message, meaning the system is simply telling you what it is doing at that moment.