Just had a somewhat similar situation with our ‘02 Atwood - a few weeks ago, it came on and sputtered for about 10 seconds then died.
No breakers tripped, no fuses blown.
Today I dug into it - after finding weird and fluctuating input voltage readings and taking the whole unit apart (needed to lube the motor bearings anyway) - I found that where the the input power wiring was wire nutted to the input power connection for the furnace appeared to have slightly melted. I took the connection apart and found it was poorly made from the factory.
Remade the connection, furnace runs great now.
Moral of the story - if you’ve checked the spark gap and swapped circuit boards - start looking for possibly intermittent wiring, loose connections, corroded or failing grounds, tight thermostat connections - and last but not least, wiring damage caused by mice and rats.
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