Short answer
Poor combustion readings — CO₂ or O₂ outside the manufacturer's range, elevated CO, or a high CO/CO₂ ratio — mean the gas/air mix is wrong or the burner is not burning cleanly. Left alone, it reduces efficiency, damages heat exchangers and creates carbon monoxide risk.
Common causes
Gas valve offset or throttle incorrectly set.
Dirty burner or blocked heat exchanger.
Flue recirculation or air intake restriction.
Wrong gas type or incorrect parameter settings.
Fan or air proving problems.
What the engineer checks
Analyse at minimum and maximum rate after the boiler has stabilised.
Inspect burner, heat exchanger and flue.
Check inlet pressure during analysis.
Repair and prevention
Clean and set combustion to the manufacturer's values at both rates, and record the results. Where readings cannot be brought into range, the appliance is classified under the Gas Industry Unsafe Situations Procedure.
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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