Short answer
High CO in the flue gas means incomplete combustion. On its own it shows the burner is not burning cleanly; combined with a flue fault or spillage, it is an immediate danger. Engineers compare CO and the CO/CO₂ ratio against the manufacturer's limits and the relevant standard.
Common causes
Incorrect gas/air ratio.
Fouled burner or heat exchanger.
Flue recirculation.
Damaged burner or seals.
Low inlet pressure at high fire.
What the engineer checks
Full combustion analysis at both rates.
Check flue integrity and room CO levels.
Inspect burner and heat exchanger.
Repair and prevention
Correct the cause and reset combustion. Where readings exceed limits, the boiler is made safe and isolated until repaired, and the situation is recorded and reported to the responsible person.
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.
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Manufacturer support
We diagnose this fault on all major commercial boiler brands:
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