Help Identify Please

Insufficient info. is the control board known bad?

Rebuilding a board generally does not entail replacing things that aren't failed and don't have a finite lifespan.

Your pictures are fairly poor so I can't make out what I'm seeing for sure, but you would generally observe some burn or severe discoloration or bulging, etc, on a failed component. What makes you feel that these need replaced?

I thought I saw some burn on a resistor or two but again the pics aren't very good.

The capacitor is 0.1uf 400V, but why are you removing parts if you don't know they are bad? Absolutely do not replace it all, these kinds of parts only fail if there is some other fault and if they have not failed, you won't appreciate any benefit. If it is very old and/or there is a switching regulation circuit then electrolytic caps ought to be replaced while you have the board out and bothering with it, but film caps and resistors, do not need replaced if they work.

Any multimeter can test resistor values. Any with a capacitor function ought to be able to measure a 0.1uF cap.

Anyway, I would have focused on the relays first if there are no overheat/etc visual signs and just replaced a faulty relay if present.

I am a little puzzled by what you labeled as R4, there are resistors that look like that but they are not used very often, normally if one has a heatsink plate on the back it is on a heatsink or else a cheaper traditional round resistor is used instead.

 

Test each component. There is no reason to assume any are bad if they test OK.

You can measure, not "worry" about things like resistors. Do test, but if you insist on buying a lot of parts then at least you have some leftover parts for a different project.

 

Contact Onan and find out what could cause the damage to the board. If you are able to mitigate that issue (lightning strike, paralleling with the local power company, severe overload, etc) then go from there. Me, I'd replace the obvious damaged parts and if that didn't work, buy another board.