Carbon monoxide is produced by burning fuel. Therefore, any fuel-burning appliance in your home is a potential CO source.
When cooking or heating appliances are kept in good working order, they produce little CO. Improperly operating appliances can produce fatal CO concentrations in your home.
Running a car or generator in an attached garage can cause fatal CO poisoning in the home. So can running a generator or burning charcoal in the basement, crawlspace, or living area of the home.