Furnace tuneup on two furnaces, both are 24 years old advise customer of this with life expectancy of 15 to 18 years, I’m checked ignition components clean burners checked electrical components and spectate exchangers found heat exchanger in main unit to be compromised producing over 10 ppm of carbon monoxide in the supply air to the house. Shut that unit off and tagged for safety. Set up designer to come out and give customer a price on replacement of the equipment