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You need a voltmeter with a built-in DVA or a standard voltmeter plus a DVA adapter. Without that, test all the other components. If they all test good, then it must be the switchbox. That wouldn't be at all unusual. They're probably the least reliable of all the ignition components with the Stator holding a strong second. But if the Stator goes, it affects spark on all the cylinders, not just one.