How does CO2 extinguish a fire?

Carbon dioxide (CO2) extinguishes a fire primarily by displacing oxygen. Fire needs oxygen to sustain combustion; when CO2 is introduced into the environment, it effectively reduces the concentration of oxygen available to the combustion process. Since oxygen is one of the essential elements of the fire triangle—fuel, heat, and oxygen—removing or displacing it causes the fire to either diminish or go out completely.

The effectiveness of CO2 as an extinguishing agent is particularly advantageous in enclosed spaces since it does not leave residues or cause damage to electronic equipment, making it suitable for use in areas like engine rooms or control rooms. Instead of cooling the fire like water would, or smothering it temporarily, the primary action of CO2 is related to the decrease in oxygen levels, rendering the fire unable to sustain itself.