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You may have bad coils in the main gas valve.
Please provide the model number of your unit.Thank you.
Ok I am uploading a service manual for this model series. It may not be for your specific unit, but the diagnostics for the coils should be the same.
Please stand by.
I'd have to say that he isnt testing the unit correctly. The control board either works or it doesnt. An intermittant fault that shows no trouble codes is highly unlikely.
Yes I am pretty sure that that is the problem.
The key fact being that if it fires up it will not fire up again until the unit cools down again. The heat causes the coils to expand. If they are going out they cease to work until they cool off again.
They are parts # XXXXX and 729 at the link below.
PLEASE CLICK HERE.
You are most welcome.
Thank you for the accept.
P.S. There are ohms values in the service manual to test these with. Please test them before you order the new ones. Hate to see you order parts that do not solve the problem.