I know, and that creates some confusion as to what input makes the valve think flame is present. I mention this because I have found that the pressure switch can make this code appear. The only things that can make the board think it s already running are -
sigal from flame sensor (never the case)
circuit board receiver for flame sense
pressure switch stuck in closed position.
I will pull the circuit board info for this model.
The wire diagram i have only have control module, gas valve and igniter. outside of that the aux. limit, fan switch and hi-limit are channel through blower relay or the thermostat.
Because it's intermittent it hard to pin point. during start up i can hear relay clicking from the control module. first click sends 24 v then drops off, again after about 10 seconds a second click 24 volt are back. this is were the ignition starts or it shuts down and I get error, 3 blinks from the control module.
To decipher just keep the meter on the 24v supplied to the module....if it drops, it is an input from a safet or pressure switch. If there is no drop TO the control but 24v drops OUT of the control then the control is bad. Then with the meter, if power drops TO the control, you can measure the 24 from the limits and then from pressure switch to see what one is dropping.
I can be more specific if you can give me the model number from the control module.
The diagram don't call out PSI and PSO.
I think the issue i the control has been changed and is not reflected in the diagram. See here -
It breaks down the inputs from the limit and pressure switch. If psi and psi do not break open on shutdown will cause this...I will leave this open for you.
Excellent find!...that is what was breaking the PSI and PSO, but instead of a true "pressure switch" it is the centrifugal switch on the draft motor. If it is at all possible to repair the centrifugal switch, it will save you a lot of money.
The motor assembly is most likely fine and it is just the switch that mounts on the end that is bad. I will see if I can find any info on them. I admire your persistance.
you are very welcome....you probably know the unit pretty good now!