As of February 1, 2011 carbon monoxide detectors are required on all levels (including basement and finished attics) of one and two family homes in Wisconsin.
Smoke detectors are also required on every level of one and two family homes.

Facts about Carbon Monoxide

A forced air furnace is frequently the source of leaks and should be carefully inspected.
Check furnace connections to flue pipes and venting systems to the outside of the home for signs of corrosion, rust, gaps, and holes. Check furnace filters and filtering systems for dirt and blockage. Check burners and ignition systems. A flame that is mostly yellow in color in natural gas furnaces is often a sign that the fuel is not burning completely and higher levels of carbon monoxide are being released. Oil furnaces with similar problems can give off an oily odor. Remember, you cannot smell carbon monoxide.

Check all venting systems to the outside.
Including flues and chimneys for cracks, corrosion, holes, and blockages. Animals and birds can build nests in chimneys, preventing gases from escaping.

Check all other appliances in the home that use flammable fuels such as natural gas, propane, wood or kerosene.
Pilot lights can be a source of carbon monoxide because the by-products of combustion are releaed inside the home rather than vented outside.

Barbecue grills should never be operated indoors under any circumstances.
Nor should stove tops or ovens that operate on flammable fuels be used to heat a residence.

Check the fireplace.
For flues that are closed, bent or blocked. Also check for buildup of soot and debris.

Check the clothes dryer vent.
For buildup and blockages caused by buildup of lint and other debris.