I was doing some maintenances on furnaces this last week and preforming combustion analysis on the systems. On this one particular day I had 2 90+ furnaces, which checked out just fine. O2 levels, CO, CO2, efficiency, and temp of flue gas. But then I did an 80% furnace and my analyzer wouldn't detect CO, CO2, but would detect O2 percentage and temp.

I swapped that one out with a different one from the shop and the new one reads all values just fine on all different styles of furnaces. Just curious if anyone has ran into this situation before, or I'd there are any guesses at to why the machine would work just fine on high efficiency furnaces and not on others.