Often called the silent killer, carbon monoxide is an invisible, odorless, colorless gas created when fuels (such as gasoline, wood, coal, natural gas, propane, oil, and methane) burn incompletely. In the home, heating and cooking equipment that burn fuel can be sources of carbon monoxide. Like smoke alarms, CO alarms also need to tested once a month. Add them to your monthly routine. When you test smoke alarms, test your CO alarms. too. Learn more about CO safety.