What Is Carbon Monoxide? June 30, 2017 Carbon monoxide (CO) is a colourless, odourless and potentially lethal gas. It can be given off by the incomplete burning of gas by household appliances, when there is an insufficient air supply - this is called "incomplete combustion". CO poisoning can occur via a shared flue or chimney, or even from a neighbour's appliance. Incomplete combustion can occur when: There is a problem with the appliance, following poor maintenance or servicing The appliance was incorrectly installed or commissioned There is a lack of ventilation around the appliance.Early symptoms of carbon monoxide poisoning include tiredness, headaches, nausea and chest or stomach pains. It is often confused with flu but if you experience any of these symptoms while using a gas appliance, you need urgent medical attention. You can't see or smell carbon monoxide, so it pays to have your appliances professionally checked.