How does a co2 fire extinguisher work?

Carbon dioxide extinguishes work by replacing oxygen, or taking away the oxygen component of the fire triangle. As it exits the extinguisher, the carbon dioxide is also extremely cold, which cools the fuel as well. CO2s might not be able to displace enough oxygen to completely put out Class A fires, making them ineffective at doing so. Class A materials have the potential to smoulder and re-ignite.