Which output is necessary to diagnose a gas furnace issue based on flame color?

To diagnose a gas furnace issue, the color of the flames is critical because it can indicate how well the furnace is functioning and whether the combustion process is occurring correctly. A properly functioning gas furnace should produce a blue flame, which signifies complete combustion. If the flames appear yellow or orange, it may indicate incomplete combustion, possibly due to insufficient air supply or an issue with the burner. This can lead to the production of carbon monoxide, which is dangerous, and may also suggest that the furnace requires servicing or adjustments.

Additionally, while other factors such as gas temperature, system voltage, and gas supply pressure play roles in the overall functioning of the furnace, they do not directly convey information about the combustion process in the same straightforward manner as flame color. Thus, flame color serves as a vital and immediate diagnostic tool for assessing the performance and safety of a gas furnace.