One of the biggest potential hazards of an improperly maintained furnace is the production of carbon monoxide. Carbon monoxide (CO) is a odorless, colorless gas that can cause death at very high levels. When a furnace isn't undergoing combustion properly, it can create CO. This CO can seep into your home if the levels are high enough or the flue pipe isn't venting properly to the outside. It is imperative that you have CO detectors in your home and business.