Combustion analysis is the instrumented test a technician runs to verify a gas furnace is burning cleanly, safely, and efficiently. A combustion analyzer draws a sample of the flue gas and measures carbon monoxide, oxygen, carbon dioxide, stack temperature, and draft — then calculates combustion efficiency from those readings. It is the only objective way to confirm what a visual inspection can only guess at: that the furnace is not producing dangerous CO and is delivering the efficiency it should.
The numbers tell the story. On natural gas, a properly tuned furnace runs roughly 3% to 6% oxygen and 10% to 12% carbon dioxide in the flue, with carbon monoxide well under 50 ppm. Readings climb into trouble as combustion goes wrong: a reading approaching 400 ppm air-free CO is a shut-it-down-now condition. Those numbers also expose a furnace that's burning fuel inefficiently long before it fails outright.
This is the practical line between a genuine tune-up and a cosmetic one. A real heating tune-up includes a combustion analysis — and a measurement is mandatory any time a technician adjusts gas pressure or the burner air, because those changes alter the burn. A 'tune-up' that is only a visual once-over and a filter swap never touches the analyzer and can't actually certify the furnace is safe. When you book furnace maintenance, ask whether a combustion analysis is part of it and whether you'll be shown the CO reading; a shop that measures and documents its numbers is demonstrating the competence these readings require.