Not necessarily true. Looks like that engine has a jumper for the drawbar connection. Removing that removed AC power going to tender board. Remove 8 pin on the engine board, leave that connector in, and apply power. If still shorted you have a bad tender board. But you can also have damage to the boiler board.
Best as Alan stated to let a tech test both boards on the bench tester with golden boards. PS-3 steam is unique in both board communicate and need to be tested separately. G