Carbon Monoxide detectors are mandated by law to be in every home, just like smoke detectors. They must also be checked once a month just like your smoke detector. There should be a carbon monoxideand smoke detector on every floor of your house. In addition, there should be a smoke detector inside every sleeping area, hallway and near the kitchen.
You should change the batteries in all detectors once a year unless you have those smoke detectors that are warranted for 10 years.