Arrived to do a performance maintenance. Set up for combustion analysis temperature split and static pressure tests. Found that the system is operating with excessive carbon monoxide in the exhaust. 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 fan in the furnace is coated in dust debris. Checked systems heat exchanger to ensure proper venting of flue gas. While checking the heat exchanger found that there is bacterial growth present on the evaporator coil. Furnace is operational at this time.