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▶ Check the gas type.
▶ Check O2 setting.
▶ 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 gaskets/seals (re-circulation).
▶ Check tightness flue gas and gas circuit.
▶ Check air way.