Thanks Lloyd, I'm having a hard time finding where they plug into the board under the playfield. As far as I can tell they only run to the controller board in the back box. Like I said, when I disconnect the coindoor harness from the controller board in the back box then the error goes away as expected. I've searched the board under the playfield and the schematics for switch 21 (the slam tilt switch that is giving me the error) and I can't find it. The wires from the switch seem to run directly to the backbox unless I'm missing something, which is totally possible since I'm definitely not a pinball tech. In the manual on the switch table, it says "Cabinet switches are read by controller board". Does this mean that the slam tilt switch only runs directly to the controller board and isn't connected to the board under the playfield? Perhaps this is a difference from the original MM?