Mold is usually caused by moisture and many new houses have mold problems as well as older homes. Many times new homes sit in the rain open during the construction phase allowing moisture to accumulate in the wood construction materials.
All materials in new homes contain moisture especially drywall,and the wood. When homes were built years ago they were not as air tight as they are today and after a time the old homes would dry out but the new homes being of a much tighter construction can take a long time to dry out and many times with the addition of water vapor from cooking and showers never dry out completely.
To offset this problem many new home manufactures install a air to air exchanger. What this does is bring in fresh dryer outside air and remove the moisture laden inside air. This is a simple decription since it also exchanges the heat from the inside air to the fresh outside air to allow for some energy savings on the loss of the already heated or cooled inside air being discharged from the home.
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HE150 Air-to-Air Heat Exchanger - Smarthome
From the sound of it you have some major problems and no one seems to care. Any CO levels that are found in any home (actually no CO level is acceptable to me) can be very bad for your health and mold can also cause many health problems also. You need to find a independent inspector that can do a ON SITE survey to discover what is actually causing the problems. This can include many items such as unvented or improperly vented bath fans. Blocked or non existent attic vent openings, improperly installed heating equipment, and hidden roof leaks just to mention a few. Then if the builder still will not take corrective steps after you present the inspectors findings your next stop is a lawyers office to force the issue. Many builders today are having a very hard time of it because of the economy and just do not want to have to pay to get your home taken care of properly.
Good luck with your problems.