I shut down the power to the GS13s & L4-Cs on BSC8 (ITMY) ISI at ~ 1430pdt.
I pulled the FeedThru Protection off as required and pulled the Field Cable from the ir side of the FeedThru.  With this access I connected a breakout board and checked for continuity between pin13 of the FeedThru to ground (Chamber, unpainted Crossbeam holes...)

I though we were home free until I got to the very last one (only 6 to do) where at Corner1 on the GS-13 I had no joy.  While there I checked the Air cable Pin13 and it was at ground...fwiw.

Chassis powered up ~1530hrs.