have you turned number on, but not let it run, (on inspection or Ind Svc.) ? I had one that when car 2 of 6 ran, it would trash all NP and IN modules on the other cars. Turned out to be a wire pulled across the brake resistor had melted and was introducing high voltage into the low voltage circuit. This happened during construction, so it was easy to find. But when you say that if number 2 is off, it doesnt fault, we dont know if it is because it doesnt run, or because the power is off. Inspection stops running, so does Ind Svc as long as CD is disabled. This allows more of the circuit in play.