I don't think the resistance test used on parallel plugs will work with series plugs. I would check the continuity thru the plugs by putting an ohmeter on plug 1 and plug 5. If you get an infinite reading, one or more of the plugs is bad.
I think you could check each plug by checking the resistance between the input wire and output wire. An infinite reading on any plug would show it was bad. I don't know what a good resistance reading is for a series plug.

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