Have seen this.
First: Onan Generators tend to be very chatty if you ask them they tell you, in a cryptic way, what the problem may be.
Story: Years ago I visited Quartzsite and the folks next to me had this very problem.. The code flashed by the generator translated to "Battery low or cranking speed too low" which we knew for a fact was not the problem.
The problem was no 120 volt.. The brushes were bad.. Other things that can cause this are a bad circuit board (Already mentioned) and a loose wire.
Thankfully for them they were parked next to the only live (part time) 30 amp outlet in the area (my RV, it's live when I'm generating) so I was able to feed 'em power but still..
I wish you luck.
I've also seen a broken bolt (Resulting in a loose connection) but that one would not even crank till I fixed the bolt.
