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▶ Check O2 setting.
▶ Check pollution Hex.
▶ Check flue integrity.
▶ Check the gas type.
▶ Check the gas supply pressure.
▶ Check the flue system; clean or repair if required.
▶ Check the gas/air ratio.
▶ Check the ratio control valve; replace if required.
▶ Check condense drain is clear.