I have a Tempstar heater that will not stay on. N9MP2100J20A1.. The fan that pulls in outside air comes on, the pilot lights and the burners ignite. After a few moment the main blower starts. It operates fine for about a minute, maybe less and then the flame goes out. Every thing then cycles down in order as expected. It stays off for ten minutes or so then repeats the sequence. I have had the problem in the past and it was condensation buildup not allowing the fan to turn at full speed. But the condensation appears to be draining fine.
What I have checked.
Running the unit with the cover off so I know it is getting plenty of air so the intake is not blocked.
Disconnected the exhaust tube to verify it is not an exhaust problem.
Removed the drain hoses, verified they are clear, cleaned the trap it is flowing clear, see water running out.
Used a meter to verify the vacuum switches are closing when they should be.
Check the overheat switches I can see to verify they are not tripping.
Checked to make sure the main blower is running fine and there is no dust build up anywhere in the heat or AC heat exchangers.
My control board ST9160B does not have diagnostic lights at least I can't find one and there is not one blinking.
I feel like I am missing something simple. Any suggestions would be appreciated.