Only runs if I hold the start switch

It is right behind the start switch. There is one bolt that holds your start switch housing down, remove it and the housing. You will see the P1 connector with a lot of wires plugged into the control board

Check the rotor slip rings and brushes first, the rings should be clean and bright

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.

The switch should be three positional.
The upper position is only for starting. Continuous running is in the middle (self holding) position. Lower position is to stop the generator.
If left in middle position it is not running you may have the switch bad, or the generator is stopped by low oil level.

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.

Check all the ground wires and make sure their not broken or loose. If the wires are good next check the neg. Wire to the coil. Too check the swiitch make sure you have continuity too all the post when the switch in the on position, this requires removing the switch.

solenoid? or relay? Good luck!!!

Standard 30W motor oil.

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.