Which issue can be identified by observing yellow burner flames?

Observing yellow burner flames is indicative of incomplete combustion, which often leads to the production of carbon monoxide, a colorless and potentially deadly gas. When a furnace operates efficiently, it should ideally produce a blue flame, indicating that the combustion of gas is happening properly with a sufficient oxygen supply. A yellow flame can suggest that the burner is not receiving enough air, thus resulting in an insufficient amount of oxygen for complete combustion.

When carbon monoxide is produced, it signifies not just inefficiency but also a serious safety hazard. Therefore, a yellow flame is a warning sign that requires immediate attention to prevent health risks associated with carbon monoxide exposure.