Hello, I can help, I had this problem on my Firepower II, You must reheat the two main boards (where the batteries go) (and where are the coil transistors, lights, etc) if that doesn't help, try also the display board.
Another problem that I see is a bad contact on the display, that is fixed with a connection jump, but it is better that first you try to reheat the cards of all the pins and then verify What happens next

In case you have already done that and nothing has worked, try to do the bridging on the display, I will show you how, only if the other methods did not work.
Another thing if your machine was off a good amount of time, leave it on for a few minutes, it can be 30-40 If the display keeps giving the error if it is already a technical fault, so try to reheat solder on the pins and then tell me what happens

Most likely your NVram has a non-displayable character in it for that digit. The last score that alternates with it shows the digit correct? No reheating or reseating of connectors needed.

Make sure the backup HSTD is set to something and then reset the HSTD.

I'd start here if you still have an original board. That's what was recommended to me.

I had some light flickering and my second digit for the ball in play was off during a game. I turned off my Firepower for an hour and when I turned it back up it's working again.