We were cold out today to provide a second opinion on an eight-year-old trane furnace. I inspected the heat exchanger for a crack but could not find one. I performed a combustion analysis and the furnace passed that test as well. I also tested for carbon monoxide which read at 2ppm. Since the combustion analysis and inspection of the heating exchanger tested fine the 2 ppm that I read with CO is most likely coming from the fireplace or water heater exhaust. I talked with customer about which carbon monoxide detector to get, and I recommended a low level carbon monoxide detector. I recommended to have one in the master bedroom, main floor, and basement main room. I also recommended to install the carbon monoxide detector lower to the floor since carbon monoxide starts from the floor and works its way up I found bacterial growth on the evaporator coil for the AC. I showed customer pictures and we talked about how to take care of that. Customer would like to wait after end of year to a