Well, the mystery is solved. There is a grounding cable and lug attached to the charging plug assembly that had an incompletely installed bolt, allowing the ground lug to vibrate to a "no-connect" situation occasionally. I think the Tesla electronics senses the "no-connect" situation and reports it as a "charging equipment fault" since electrically it looks like the ground connection from the UMC, HPWC, or SC is electrically open. The mobile service tech fully fastened the bolt, problem solved. Took less than ½ hour.
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