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and thanks me to help you today. I am a licensed diagnostic and repair technician, I look forward to working with you.If the lambda readings are too high the system will be pulling in air somewhere or running too lean. The first step will be to fully seal the exhaust system to make sure there are absolutely no leaks.The Co2 being high would point to running rich, or a faulty catalytic convertor, which contradicts the high lambda and the fact you have P0420 would point to a bad catalytic convertor. The o2 sensor could cause this but I don't think the lambda would be as high as it is if the o2 sensor was causing all of this and i would expect a different fault code relating to the fuel system mixtureI think its quite safe to say that the catalytic convertor needs replacing and the leaks need fixing, but it is good practice to replace the 02 sensor with the cat anyway so i would highly recommend you also replace this sensor. The car should go through emissions with no problems once this is done.