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When was the last time you had the LP pressure at the tank regulator checked and adjusted? That would be the first place to start if you haven't done that in the last year. Some are adjustable and some aren't. If you need a new one I would go with a Marshall as they are about the best one out there. About $40 or so. Once we rule that out we can go further as all follow ups are included in our fee.
When you say "serviced"......what all did thy do? Many shops never bother to check the LP pressure. If this uses low pressure LP, you can't just take it out of the tank directly. It has to be at the correct pressure.
Pretty strange...........I will go ahead and opt out of this question as we have a really good Onan tech here that works on these for a living. I suspect he can get this sorted out for you. I don't know if he is around this week-end or not. I will keep an eye on this as I am curious as to what he might think is the problem. I don't know any good reason for this not starting when that is all you did.
What is the model number of the genset?
The single most common cause of this issue is that the set is no longer producing electricity.Remove the control panel from the front of the genset.Set your meter to ACV.Connect the black lead to a good ground.Connect the red lead to the back of the circuit breaker (either one), on the terminal farthest away from you.Press and hold the start button for 6 - 8 seconds, and note the voltage.How much voltage is there? .
If you are getting 140V at the breaker, you need to connect a frequency meter to the circuit breaker and test frequency while it is running.
What frequency do you get?
It is very possible.
There is a diaphragm inside the demand regulator that can get dirty or stiff, and create problems.
But unless you have had gaseous fuel training and have the proper equipment, this is not something you can test yourself. The regulator is not inexpensive, and I really don't recommend replacing it unless it has been tested.