My copy of the Hydro-Hot service manual says to remove the coil, disconnect the wires, and with the burner head out, apply 12V to the yellow lead and negative or ground to the brown wire and a spark should appear across the electrode tips. I just did this on my unit and no spark. I metered the coil with my digital meter and it shows an open coil. I am hoping that a new coil will also resolve my ignition issue.