Those Duo-Therm furnaces are no longer made by Duo-Therm. I just pitched my center furnace. You can get the UIB boards from Dino Boards(?) for around $100. Service is too expensive to repair an 18 year old furnace and new ones are $500. I feel your pain!
Check your in-take and out-take tubes. I had large bee hives in mine that blocked air-flow (although they still didn't work after I cleaned them out). Oh, and bad words form in your head regarding the 345 designers while removing the furnaces! Careful not to let them slip out.