Worcester Bosch electrode for use in Greenstar and Greenstar Er P boilers. An ignition (spark) electrode causes the spark that begins combustion within the burner while flame sensing electrodes are responsible for checking if there is a flame present in the burner to make sure that the boiler doesn't keep supplying gas without a flame. This is a spare part and must be fitted by a certified Gas Safe engineer. Worcester Bosch Spare Part GC No. H26-544.