Some of the things that can cause it to die when you release the botton.
Low oil switch
Bad brushes/slip rings
Bad control board
I'm not sure about the power source for the ignition system but I do not think the poster was correct on that one. I think it always either self-generates (Magneto) or burns battery power. There are advantages to both systems.
The onan bypasses several fault sensors (like the low oil switch) when starting but when yor let go it checks them
One of them is the output frequency and voltage... Unlike those el-cheapo "Contractor" generators. the Onan monitors itself, same as a Surge Guard monitors the park's shore power, and if the voltage or frequency are beyond what is is programmed to accept.. It shuts down
A friend got that message when his did as yours.. and he had to do a brush replacement.. This happened when he was parked in the land of no shore power.. However lucky for him, there was a TT-30 outlet that was live at least part of the time near by (It is in one of my basement compartments in fact, he was next to me).
