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I still maintain that I would not replace any components until the Oxygen / CO2 levels have been checked. Typically the Oxygen level should be as close to zero (0) as possible and the CO2 should be around 16% (from memory). A pre-test MOT emissions check should reveal the result. If the oxygen level is much higher than zero then you are drawing air through a poor weld, split or crack in the exhaust system or a poor joint/gasket and the result is what you are describing