That's if you for sure have a non-intermittent open/short. Some components can fail due to temperature coefficient due to cycles of operation, ambient temperature or the combination. For instance, if a primary winding has a microscopic fracture/opening that conducts until the physical expansion of material and then the opening becomes non conductive. This would give an intermittent failure. So you would test the component against known values and it would pass because it is not in the same environment and needs to expand to fail.
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Well said. Heat and Moisture are a huge deal!
Keep in mind that even if they test good electrically most of us have no way of testing the dielectric strength of the assembly. Meaning, electrically its sound and tests fine, but its the casing material which is breaking down allowing the energy a path to ground vs a path to the plugs electrode.
Get a visual on the coils, wires, caps in the dark looking for arc-over or use a spray bottle to mist water droplets on the suspect area to "change the conditions" and make it act up! If a light spray of water causes a misfire, its just a matter of time before failure occurs.