I/O board has only 1 red LED (Flex Check) and the other 7 are green. You will have to verify which one doesn't light up to diagnose the issue. I've posted some time ago the repair manual for the 2013 Mac Pro.
you will need a magnet to have all LEDs light up on the I/O board.
If the repair manual is not there anymore, let me know and I will get it to you. I also have most parts available for sale if you need anything in particular.