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This can be caused by any number of things. Do you have the Onan Service Manual for this?
It might be the control board or the controller. Several things can cause this. Batteries up in good condition and fully charged? And do you have the Service Manual or not? I am attaching a link to the Flight Systems diagnosis sheet for you. Print that out so you have it. https://www.flightsystems.com/troubleshooting/
Then follow the trouble shooting guide and manual and see what you can rule out. Flight Systems as some superior parts for boards and controllers and I use them exclusively for my customers. You have to track this down one item at a time.
That guide is good one and combined with the Service Manual you should be able to narrow this down........at least to the point of having some of the electronics tested. They give you some of that.
Get back to me if you need more on this once you have some some testing.
As I said..........there are any number of things that can cause this. You have to test to find out which one is causing the problem. Some of the electronics a shop would have to test.
Just checking back in to see if you were able to resolve this issue or do you still need more input from me here?