Anan Gen. III not working

Many Onan generators are smarter than many humans.. For example.. If the generator is not working (That is putting out power) why keep burning fuel?

Several things will cause a shut down,, Frequeny out of range, Voltage out of range, Low oil All will cause a shut down,, these sensors are BYPASSED when starting.

Bad connections on the control board can cause false out of range readings

Bad control board can think normal is out of range.

And this is only some of the things that can do it...

One thing that may help is reading the error code.

On, one other thing.. You said only the dash switch starts it, the "local" switch, I gather does not.. this argues strongly for bad connections.

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It is a 6.5 kw gas generator, with 303 hours on it. The rig has 3/4 tank with new fuel. It runs fine until you release the start switch. Stops immediately when you release the button. I have held the button down for 15 seconds and ran the as long as I held down the button. I drove the rig home from the lot and several times at home. The fuel is good. Oil has been changed and at the right level.

Onan generators are used in many RVs and have a pretty good record. Yours is probably 4000 Watts. Most problems come from old fuel left sitting in their carburetors and gumming up the works. Some people have had good luck using Sea Foam to get the carburetor working and some have had to get a new carburetor. Another possibility on a vehicle that old is the rubber gas line to the generator may have cracked and is letting air into the line. Older rubber lines don't hold up as well with the alcohol in modern gas. You also have to have 1/2 tank of gas. Generators draw from the top of the fuel so you can't run out of gas while camped. Never try to run the generator while plugged into shore power. You could feed back to the lines and kill a lineman if he is working on a dead line. Transfer switches should eliminate that but anything can fail once.

If you are pugged into shore power this might happen as the genny senses the shore power. Try it without being plugged in.