Have Onan 3.6 KY propane gen that will not keep running. Runs only as long as start switch is pushed. Then stops immediately after release. Fault code is 15 (underfrequency) and suggests removing heavy loads - but load isn't normally connected until about 30 seconds AFTER gen has been running. Same behavior when load switch is shut off at generator. I understand the unit won't start until AFTER the code is cleared, but manual doesn't tell how to clear the code. Tried disconnecting battery for 1 minute - code 15 still there. Have read to try holding stop switch down for several seconds to clear codes (haven't tried that yet) - but several readers say that doesn't work either. How can I clear the fault code to see if problem (fault 15) is still there??? What could be causing this problem with no load on the generator?? How can I fix it?? Thanks, Jack

what model is it KY KV need that and Spec. If it blinks 3 times it indicates a lower service fault. While blinking hit the stop button 3 times fast, then read the code. It will blink, pause then blink again and repeat. What is that code and I can help you. IE 4 blinks pause and 3 indicates code 43

Is this gasoline or propane fueled?

Is the generator coming up to proper speed? A cycles (hertz) (should be 60 in USA) meter will tell.
If it is up to speed, see if a UPS unit is made powerful enough to power the microwave. That will handle the initial starting load from the microwave.
UPS units are handy to power stuff while you are getting the gen going.

There can be numerous reasons why and is difficult to diagnose without more information. Assuming plugs filter etc are fine and new or clean and gapped. One thing I would check would be the LP pressure going into the unit, Then check the demand regualtor. You did not mention if it keeps running when it finally starts. Over time the demand regulator can accumulate a heavy wax preventing the diaphragm from opening. This wax is the result of a lubricate LP gas companies have in the gas. It can make starting harder, then result in it not starting at all. Regulators typically should be replaced and not serviced due to the high possibility of gas leaks. Hope this helps www.advanced-power-technologies.com

Check rotor slip rings for cleanliness, brushes, stator resistance, rotor resistance. Then replace control board if they check out.

You say Guardian that is Generac???? Then you indicate Onan 4.0KY. If it is a Onan and it stops after you release the button there are a couple of things. 1 st check the brushes, and polish the rotor slip rings, check the resisitance of the rotor, the electrical harness plugs are tight. The control requires a voltage feedback while cranking to engage the PCB run circuits, if all those check out you have a bad control board.

More likely the switch is bad, and only make run contact when in the start position.
Easy to test the switch continuity with a ohm meter when it is not running.

Possible low oil / low oil switch

Governor is out of adjustment

Altitude settings are incorrect

Idle control circuit

Cleaning the brushes is used to address an AC output problem. If the generator would run for a few seconds after letting go of the button, then this might apply.

But, since yours is dying as soon as you let go of the button, that tells me that you are probably looking at a low oil condition, or a sensor / sending unit failure for oil level.

Without spec number, hard to remember if the sending unit is for low oil pressure, or low oil level. Either way, the ignition is being grounded. First, double check your oil level, make sure generator is reasonably level. If still not resolved, then locate the sending unit, and remove the wire that is on the end of it. There should be a connector either a few inches from the sending unit, or a few inches from the control board (coming from big white plug). Disconnect at either place. Try starting again. If it starts this time, double check oil levels. If those are fine, then sending unit has failed, or oil pump (if pressure unit) has failed. In either case, don't run for very long until sending unit is replaced, just in case.

Flight systems can test the onan circuit boards. google them they will come up. First class operation my board was was checked out and back to me in about a week, They also sell new boards and deduct the 20 dollar bench fee if yours is bad and you buy a replacement from them.