I believe F4 on Glow Worm boiler is "Flame not detected" maybe the flame sense probe faulty, the igniter faulty or the flame detector module pcb.
With the power turned off, check the
continuity between the flame sense
electrode and terminal H2 . 4 on the
control board (PCB). Check the
connector on the end of the wire.
Check that the spark electrode is
pointing towards the flame.
Check the spark electrode. Change it.
Sid
October 2007