Before you test any equipment at an existing installation, before you even perform a clean and check or other maintenance, you need to check ambient carbon monoxide (CO) levels before entering the home. Run the equipment through a complete cycle unless you are there for a no-heat call.
If at any time during this test, ambient CO levels exceed 100 ppm, evacuate occupants and ventilate the dwelling immediately. For lower CO levels, consult the "Ambient CO Limits" section of the Testo Combustion Guide or consult a local authority with jurisdiction over these matters.