1mre

Hi! I have a 2016 S3 with intake, catted downpipe, exhaust and tune and am currently chasing down emissions issues. All my readings were well above the normal values but interestingly the lambda value they measured off the exhaust tip was ~1,4 which is too lean. To me it seems like there is additional air getting in somewhere after cat that messes with readings. Can this potentially cause higher CO and CH readings also?

When checking O2 sensor readings with vcds I can see that the upstream wideband sensor shows ~1 during idle and cruise so thats good. But downstream sensor shows erratic behaviour changing from 0,2 to 0,8 and back every couple of seconds during both idle and drive. I can hear the exhaust sound also change when the O2 sensor value changes. Can this behaviour also be attributed to the possible leak or what could be the reason for my troubles?