That board failure would either be a PV diode shorting or opening, or a fet for lights. Not sure how you are testing it with a PS-2 3V test set since there is no provision for that. What is the engines issue when that board is installed. It is the Receiver MUX in the engine. The Tender has the transmit mux.
If it has issues usually it is a light problem. Nothing to do with motion other then it does provide the return for Tach. G
I used process of elimination. Removed the PS2 3v and placed on my board tester. Board tested fine. Placed back in tender. Problem. Began disconnecting connections in engine, post mux. Same problem. Disconnected mux. No problem.
its definitely the mux in both locos.