Onan 5500 Starts then stops after releasing button

If you suspect the circuit board is faulty, before tossing it ($$$), try repairing it. Most of these boards suffer from vibration related issues. Pull the board, and using a good light, a magnifier, and a small soldering iron, look for tiny cracks in solder connections, especially around connector pins and relays. Resolder anything that looks suspicious. That may be all that's wrong with it. What have you to lose?