If your Onan Gas Generator will not start. ( see also, Generator / Fault Codes )
1.) First, is it getting gas? Remove the rubber fuel line hose at the carburetor and try to start it. Fuel should be spurting out of the hose.
2.) If you're not getting fuel. It may be a bad fuel pump which is located in the Generator Compartment. If you hear a sharp clicking instead of a dull thuding sound when you are priming the generator, it may have a bad fuel pump. It is natural to hear a sharp clicking when you first start to prime the generator, but if everything is working correctly the sharp clicking turns to a dull thuding sound when fuel reaches the pump.
3.) Is there a spark to the plug?
4.) If your generator has sat idle for several months without running, and it turns over but won't start, the gas could have varnished up inside the Carburetor. There is usually No Cleaning the Carburetor, it has to be replaced. Periodic use of a Fuel Stabilizer that keeps the Fuel Fresh is recommended by Onan.
5.) Does it start at the Generator but not from the Living Quarters if so equipped with an additional start button? Then the problem is between the Generator and the additional start button inside the LQ.
6.) If your Generator's been running ok and it quits and it won't even try to start again, the Battery might be Dead. Check the Water and then Charge the Battery. If it starts, and if the battery is no good, it will run till it's drained the Battery and then it will shut down again. The problem is the Battery is not recharging while the Generator is running. The Generator needs 12 volt DC to run. It does not recharge it self and it won't keep running if you disconnect the battery. ( see also, Electrical > Converter > How do I troubleshoot the Converter )
7.) It could be that the Positive and Negative Cable on the Battery needs to be cleaned to make a better connection. Simple, might try this first. Just because the cable is on the battery post does not mean it's making a good connection.
This is how the Battery that starts the Generator charges:
The Converter (if equipped) inside the Living Quarters acts like a Battery Charger. 120 volt AC comes from the Generator to the Converter, then 12 volt DC goes from the Converter to the Battery that starts the Generator.
( see, Electrical > Converter > How do I troubleshoot the Converter )
8.) If your Generator's been running fine, and if the next time you try to start it, you just hear a clicking sound. Remove the Negative Battery Cable from the Battery that starts the Generator, and you can do this at the Generator also. With the Panel Door off the Generator, use your left hand and reach through the Generator on the Left Hand Side until you find the Fly Wheel. Spin the Fly Wheel to make sure it's Free. Then hook up the battery cables and try starting again. Sometimes when a Generator shuts down, it stops at an exact point at what I'd call Top Dead Center. You just have to move it slightly off top dead center and then it's ok.