The only thing that lights up the ERR light on a BOSS machine is the ERS board. I believe the BTC-1 has an RRS. This board does communication to the outside world and it has the PROMS with the BOSS (Bridgeport Operating Software System) on it. So if the machine does not have access toits basic operating system, it will not function correctly.
I suggest you make sure all logic voltages are correct. Remove each board and clean it and clean the edge connector with a soft eraser. Make sure it is seated well when reinstalled. Make sure all fans are running.
George
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)