Carbon Monoxide detectors are mandated by law to be in every home just like smoke detectors are. They also have to be checked once a month just like your smoke detector. There should be a carbon monoxide detector on every floor of your house and a smoke detector at the least on every floor also. It is better to have a smoke detector inside every sleeping area, hallway and near the kitchen.
You should change the batteries in each once a year unless you have the new smoke detectors that are warranted for 10 years.