Final answer:
A high CO₂ reading in an oil-fired furnace indicates too much excess air.
Step-by-step explanation:
A high CO₂ reading in an oil-fired furnace indicates that there is too much excess air. When preheated air is blown into the furnace, the coke in the region of the nozzles is oxidized to carbon dioxide (CO₂). The hot CO₂ then passes through the white-hot coke, where it is reduced to carbon monoxide (CO). This reduction reaction is: CO₂(g) + C(s) →→→ 2 CO(g).