Armstrong GUK125D20-2B won't ignite

What type of ignitor you have is it a glow type .if yes does it glow if it doesn't glow you have a safety switch preventing the unit for unhinged properly.
Check all your safety which is pressure switch, roll out switch, limit switch

Check pressure switch and hoses to it . check air filter is clean . check you have gas in the line . check connections on gas valve check flame sensor

Generally sounds like they are thermostats

Have you checked the flue inlet and outlet to see if it doesn\'t have a wasp nest or bird nest or rocks. check the hose that is attached to the pressure switch and the port on the collector box or inducer fan motor to see if it is clear. if you take the switch off and lightly suck or blow into the port it that will close the switch. If you have a manometer you can check the pressure differential. On the pressure switch it will say what pressure in will close at ...Example .52" WC. or 1.25"WC You would attach your manometer to the tube attached to the pressure switch and start the combustion blower to read the pressure. If you have over the reading and the switch didn\'t make you will need to replace the switch. if you have less than you have to make sure the flue is open. You can check the flue with a simple trick by unhooking the flue pipe from the combustion blower and then check the pressure and you may have to unhook the air intake also. If that doesn\'t work you could have a plugged secondary heat exchanger...... Good Luck.

Hit the side of the furnace. Some times there's a sensor inside that gets locked up from a dirty filter.

Hello, sounds like the relay on the intergrated control board is sticking. This will require the board to be replaced.

http://www.armstrongair.com/armstrong-links/

from here select the particular furnace that you have this is the only resource the is avail. due to the furnace being a Lennox made furnace

you will have to change the pressure switch no qouick fix with this sorry