Can you tell me where does carbon monoxide come from in your home?

Combustion: Incomplete combustion yields carbon monoxide instead of carbon dioxide--so anything that burns--gas water heater, gas stove, running car in garage, propane heater, fireplace, candles, etc. This is why the venting / exhaust handling is so meticulous and should be periodically inspected to be sure it is not obstructed, and service checks for operating efficiency so as little co as possible is made.