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Are you saying you do or do not get 24v at valve?
Ok if you do not get voltage we really should get a different flash code at that point. Possibly a bad board. Take a close look at the igmitor and make sure it is not burned and is lighting completely.
Nothing between the igniter lighting and board sending signal to gas valve.
You are correct.The furnace should only attempt to light between 3 and 4 times and then go into lockout.
The only way you could energize valve is o run a wire directly from R and C at board ,but this would be merely for a test and either have gas turned off just to hear it click or make sure that igniter is hot so burners light immediately.THIS IS DEFINITELY for TEST only to verify valve works, but Valve is NOT the problem because we are not getting signal to it.
And yes you are right. Limit switch or pressure switch are neither one an issue or we would not get to igniter stage
Honestly I do not.I just know the board should be signaling the gas valve which it is not and if we do not get ignition, board should go into lockout and give us an error code and it is not.
Only other thing I can thin of that may cause this is a poor ground. I have seen a poor ground cause erratic issues.
But yes my suggestion once you verify ground is good would be to replace board as there are no other components in circuit between igniter and gas valve opening.
And you are correct, transformer would not send 24v to allow anything to happen if it were bad.
I thought the low pressure gas witch was only used with LP to prevent valve from opening if lp pressure was low.(Didnt think to ask if you were on LP) located before the gas valve in the gas line and wired in as shown in schematic.
With your expertise in the computer electronics, you can possible repair the board and that is something I admire you for.You might be able to do a Google search or contact White Rodgers or whoever manufacture of board is and find or get them to send you a data sheet showing which resistor, diode,fuse ,transistor etc controls and just repair the one burned out or shorted.As an HVAC contror, we just replace the board.