Responded to emergency no-heat call for 10+ year old natural gas furnace previously shut down by gas company due to dangerous carbon monoxide readings. Diagnostic testing with Testo analyzer confirmed critical CO levels exceeding 6300 ppm (14,000 ppm air-free), indicating severe heat exchanger failure. Inspection revealed significant secondary heat exchanger clogging, rendering the unit unsafe for operation. Recommended and scheduled complete system replacement due to age of equipment and non-repairable safety hazard.