I have an old Amana AC unit with Gas Furnace.
It has a White Rodgers 50A51-225 module.
I was about to trash and replace the entire unit but managed to get the AC working.
The problem is that the gas furnace won't turn on.
I don't want to spend the $200. on this since I'm borderline about replacing it,
so I'm not sure that a new module is necessary, or if it is, that I would buy one for this.

I understand that the sequence is that the fan must come on first,
which activates the pressure (vacuum) switches, which in turn enables everything else to power up and ignite the burners.

The problem is that the induced draft fan does not turn on when the thermostat is switched to heat.
I get either two or three flashes, indicating pressure switch open or short.
The induced draft fan has two speeds and the motor runs when I give it direct 120vAC on the high and low wires.
There are two different pressure switches that tap off of the single tube.

Can somebody answer the following questions?
Why does the induced draft fan have two speeds.
What speed does it usually run at?
Why are there two pressure switches? (My suspicion is that the vac has to be enough
to trip one, but not enough to trip the other..)

If the fan doesn't turn on when switched to heat....
is the module bad?
I'm thinking that there has to be a relay on the module board that does the switching....
can that relay only be replaced?

Is there a way to bench test the module?

I hope that's enough info..
Thanks in advance for any help....