First, I need to warn you that working on this yourself, you are going to void the warranty, and you will be responsible for the cost of any parts.. There is a good chance that the controller is what is wrong, and it is a somewhat expensive part.
Second, for a Code 15, there are some things you can check, and there are some things you can not, unless you have a special tool from Onan.
The reason that the unit is running fine with the light flashing is because at that point, the genset is in ERO mode. The genset is not producing power, but the engine will run. This is because your unit is considered a commercial unit.
Commercial units are often mounted in bucket trucks to run the hydraulics. Even if the power side fails, the hydraulic pump must keep running to get the bucket down.
If you wish to continue with this, I can help with what you will be able to do. A frequency meter will be required.
If you wish to continue, I will just need confirmation from you that you understand you will be voiding your warranty.