What make/model furnace is it? The common Atwood/Hydro Flame 8500, 8900, etc, series are almost entirely serviceable through the outside panel. The "motherboard", more commonly known as the "ignitor board", is easily reached on the upper left side in a conventional horizontal installation. Loosening a wingnut and disconnecting a couple of plugs is all it takes to remove the board for repair/replacement.
Dutch
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