Dirty fresh or combustion air intake screen similar to the one in this picture will cause the furnace to not get the proper air it needs to run, Because of a negative pressure in the room. This negative pressure will cause air to be brought into the room through the only duct to the outside which is the furnace exhaust vent. The furnace will have a back draft safety switch to protect the home owner from carbon monoxide poisoning and when the safety switch trips it can be easily reset, But once reset the reason for it triping in the first place must be addressed .

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