Originally Posted by Dustdevil
The control valve for the propane line to the burner has to see constant pressure from the supply tank before it will allow the valve to open, remain open, or continue to supply gas to the burner(s). If not, it likely won't allow the blower to start either. If you turn off the tank outlet valve, pressure eventually bleeds down, and the control valve shuts off. Turn it back on again, with the theoretical short circuit showing a demand to start, and it will start up again. That's why a short circuit will cause this, just as if you had turned on the thermostat and set the temp high enough to turn the system on.
The opening and closing of the gas valve is controlled by voltage from the circuit board, which actuates the solenoid. Whether there's gas pressure or not wouldn't affect the state of the valve. It would be opened or closed depending on the presence of a control voltage. And if the control circuit is working properly, the valve would only open if the fan is running, and the sail switch has been actuated for a specific period of time.
A lack of propane pressure wouldn't "tell" the furnace control circuit anything. The only way the furnace would "know" there was no propane would be a case where the ignitor was actuated, and no heat was sensed by the flame sensor.
And if the flame sensor hasn't proven a flame after the ignition cycle, the control circuit will shut off the gas valve, requiring a reset (off/on) before the furnace is able to operate again.
I still see no way that propane, flowing through an already opened gas valve (which shouldn't be the case in a non-running furnace) could cause the blower to engage and the ignitor to activate.