Quick chemistry lesson: CO is carbon monoxide, a colorless, odorless gas produced when various substances are burned (auto fuel, wood, whatever fuel your boiler runs on) that will kill you. It bonds with your body's red blood cells much more readily than oxygen, which the red blood cells are supposed to bond with, so you suffocate as the CO crowds out the oxygen in your system.
CO2 is carbon dioxide, produced when the same substances are burned more completely. AIUI, plants take in CO2 and give off oxygen, animals breathe in oxygen and give off CO2. Too much CO2 in the atmosphere is agreed to contribute to long term climate change, but CO2 itself is not toxic.
I seriously doubt that your fire alarm detects CO2, but it would be a good thing if it detected CO.