A CO2 detector is mandatory on a new build if you have a attached garage or a fuel burning appliance such as a woodstove/fireplace, gas furnace, or other gas appliances.
If you have a older house with hard wired smoke detectors, you can install a combo CO2/smoke detector in its place.
Remember CO2 is a oderless, tasteless. IT IS OFTEN NOT DETECTED UNTIL THERE ARE SYMPTOMS. It does kill
Make sure you have some type of a CO2 dector.