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I have an issue with a gb142 setting 6a fault. Had a broken Venturi which I replaced. Gas pressures all checked within specs Cleaned as good as I could but manual states to spray exchanger with mineral oil then high fire. Well. The issue is it doesnt fire so this won’t work. If I block air intake tube it fires for about 15 seconds then goes out. Buderus says it’s dirty but from what I compared to this unit seems rarely clean. Any help?