Arrived to do a performance maintenance. Set up for combustion analysis temperature split and static pressure tests. Determined the cause of the elevated carbon monoxide in the units exhaust is caused by the cleaning supplies, paints and litter box. Verified operation of all mechanical and electronic components of the furnace. Checked blower for cleanliness and to see if there is any play in the bearings. Found that the furnace fan is coated in dust debris. Checked systems heat exchanger to ensure proper venting of flue gas. Checked ductwork for cleanliness. Found that the ductwork has a heavy coating of dust debris. Spoke with homeowner about the system. Furnace is operational at this time.