What would a stack gas reading indicating fair to good combustion look like?

A stack gas reading that indicates fair to good combustion would typically show low oxygen levels and high carbon dioxide levels. This is because in an efficient combustion process, the fuel is burning relatively completely. When the combustion is good, the presence of CO2 indicates that the carbon in the fuel is being converted effectively into carbon dioxide instead of carbon monoxide, which is a sign of inefficient combustion. This aligns with the concept of stoichiometric combustion, where there's enough oxygen to react with the fuel completely.

In the context of the other choices, a high oxygen level typically suggests that there is excess oxygen in the combustion process, which may indicate incomplete combustion, as not all the fuel is being burned. High carbon monoxide levels signal incomplete combustion or insufficient oxygen, while equal levels of oxygen and carbon dioxide also do not reflect well-balanced combustion. Thus, a low oxygen and high carbon dioxide reading is consistent with effective fuel burning.