Hi. In the vast majority of furnaces the system is on them and this means that they must be removed from their niche to repair them. It is usually a small black box with connections in sight, that is the generator. The switch is on the same axis of the robinete, the key you use to turn it on. There are models with the ignition apart. What you should control before disarming something is the wire that comes out of this spark gap and goes to the sparkling electrode. If this is deteriorated, the spark will be derived to any metallic part of the furnace and not in the spark gap. The connection of this cable with the spark plug would also have to be checked. regards