Carbon monoxide (CO) is a gas by-product generated by an incomplete burn. For example, a faulty furnace, a leak on a gas clothes dryer, or the back-draft from a gas fireplace. It is colorless and odorless, toxic and flammable. Your CO detector is expressly for the purpose of detecting this by-product. IT IS NOT DESIGNED TO DETECT A NATURAL GAS LEAK IN AND OF ITSELF.
What’s the difference, you ask? Natural gas is not a by-product. It’s the fuel on which your appliances run. A large enough leak should be noticeable because of the mercaptan, a non-toxic chemical with an obvious smell (often compared to rotten eggs) that is added to natural gas for safety reasons. However, a small leak can cause gas to build up over time, raising the possibility of fire and explosion before the odor is noticeable.
Consider adding a layer of safety to your home by installing a natural gas detector in addition to your existing smoke and CO detectors.