Short answer
The CO/CO₂ ratio is CO (ppm) divided by CO₂ (ppm) — or CO ppm ÷ (CO₂% × 10,000). It shows how cleanly the burner is burning independently of excess air. A ratio above the manufacturer's limit indicates incomplete combustion; as a general reference, values above 0.004 on many modern appliances need investigation, but always use the manufacturer's figure.
Common causes
Gas valve set too rich.
Burner fouling or damage.
Flue gas recirculation diluting combustion air.
Analyser probe placed incorrectly or uncalibrated analyser.
What the engineer checks
Confirm analyser calibration and probe position.
Re-test after stabilisation at minimum and maximum rate.
Repair and prevention
Correct the gas/air setting, clean the burner and repair flue faults. Record ratios at both rates on the service record.
Figures are typical for UK natural gas. The current edition of the relevant IGEM standard and the manufacturer's instructions always take precedence on site.
Need this done on site?
Manufacturer support
We diagnose this fault on all major commercial boiler brands:
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