Perform maintenance on gas furnace in preparation for home sale. Inspected all major and critical components including blower motor, blower wheel, inducer, ignitor, flame sensor, burners, heat exchanger and gas valve. Tested Co levels at a flue pipe and found they were a little high at 108 parts per million. Did a visual inspection on heat exchangers and found small crack in heat exchanger. Tested Co levels out of the vents and we're at zero but heat exchangers are cracked. All other operations are normal at this time. Based on age and cost of repairs needed and potential cost of future repairs I recommend replacing unit. Will provide estimate of repairs and replacement.