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Hi Ben, I am Kevin....... Can you check the voltage potential across the pressure switch during an attempt for heat?
that is a good thought on the pressure sensor Ben, I would agree if the piping "intake and exhaust" was not installed correctly, sized correctly, distances on termination and height and certainly in the same plane, (same side of the house).
I was looking at it more service oriented because you said you had 11.8 volts on the gas valve, when you have a 24V circuit energizing that... Why would that voltage be that low? Maybe because the contacts on the pressure switch or one of the limits are not closing or corroded just enough to create a resistance between the contacts. Hence the thought of checking the voltage potential across the contacts while energized, and hopefully you can do that to the limits as well and verify where we are loosing the power and can then go in that direction for parts or correction.