Model: MPGA100A4
Serial: 5004879510
Unlabeled number under the serial number: 20093403A
Natural Gas

Hope this is all the info about the unit you need. It's a Ducane furnace, and last season, it would ignite, run for a few seconds, then go out, and just do that over and over again indefinitely. There is an in-line fan in the exhaust tube that is always on, I guess to help pull the exhaust outside. There is also a flap valve this in that same exhaust tube that could be adjusted to let normal air in too. It is between the furnace and the in-line fan. I adjusted this to full closed position and duct-taped it so no air could be pulled in, and hamper the fans ability to pull the exhaust out. (hope this makes sense) This let the furnace run for the rest of the season without issue until the duct tape fell off, or a spot opened in the tape to let air in. Its been of the entire summer and now that its getting cold here in Michigan, I turned it on, and now it will not ignite or even try to, no sound comes from it except the blower inside the furnace, and the in-line fan. The gas is on, I did the sandpaper on the sensor trick, and nothing. I cannot smell any gas or anything. If the thermostat is all the way down, the only thing running is the in-line fan. When I turn up the thermostat, the blower inside the furnace will come on, so I know its getting information from the thermostat, but nothing else happens, no ignition, no igniter getting red hot.. nothing but the blower. It has a honeywell thermostat/valve. Model number for that is: SV9501M2056

Here is a link to the exact one that's on it:
SV9501M2528 SmartValve Honeywell FREE NEXT DAY AIR SHIPPING

I'd greatly appreciate any help that can be given, I would like to avoid a service call as my financial situation is precarious at the moment, and will be jobless for a year or more come November (Yay for General Motors... )

The Honeywell Smart (DUMB) valves have some problems especially where the wires are on the valve. Try moving the wires around while the unit is calling for heat and see if the furnace will start working as a first step. Post back with the results.

BTW the inline fan is not supposed to run all the time just when there is a call for heat.

I just went and tried it, and no luck. Blower and fan come on, but no ignition or click or anything. :(

OK now you will have to start tracking down the 24 volt circuits etc.

Here is some info for you to use.

Thank you for the info, I will run through it and grab a voltmeter when I get up tomorrow, hopefully I can resolve this with your help! Thanks again

Also, the problem I had last year, that I had to block off that air bypass in the exhaust tube for the furnace to quit cycling on and off. Is that sort of thing normal, or could that have caused some of my present difficulties?

Is that sort of thing normal

No it is not normal. I suggest you contact a local dealer for a on site inspection of your furnace. While I have no model number information on Ducane which I believe is a Lennox company the bypass you blocked off does not bode well for the proper operation of the equipment.

Ducan furnace model# fpbb075a3b it want light

Ducane furnace # fpbbo75a3b want light