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Monday 19 March 2018 at 10:51
Also check the paper insulator behind the start button leaf blade. They do wear out and will make contact with the metal support tab behind the switch and will ground. Coin door frame is connected to ground as long as all the grounding hasn't been touched or is corroded.
Source: http://pinside.com/pinball/forum/topic/creditstart-button-adds-more-than-1-player#post-4288490