If you frog the lines (swap 1 and 2) and the problem doesn't follow the line but stays on the same co port then you most likely have a dead port. This could be caused by a loose cinch connector, a well known problem. Make sure it is nice and tight on the side of the control unit. If is tight then If you have a free port I would simply shift the lines to say ports 2 (1) and 3 (2). The STS has the first 4 co ports are on the mother board so the system would need to be replaced or sent for repair if the port is actually dead.

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