Coleman mobile home furnace...Model number

Question

Coleman mobile home furnace...Model number DGAA077BDTB...The furnace about a month ago started going out ( flash code- Combustion Air switch stuck closed)We replaced the thermostat on the wall and it stopped doing it and ran fine then last night it stopped again ( Combustion air switch failed to close) and when i got up the furnace was working and now for the past few hours it refuses to do anything...I am on a very very tight budget...and need some idea of what is wrong and do i need a professional to fix it and is it going to cost lots of money??? Help me please...Than you