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      03-05-2018, 08:30 PM   #15
drivef30
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Drives: 17' 340i xDrive M-Sport
Join Date: Aug 2017
Location: USA

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Puts the o2 sensor futher away from the airflow, causing the sensor to pick up less unburnt fuel (I believe no2,co)... orginally, the cats is designed to turn those gases into N2 and CO2... When putting the sensor further back it, the o2 sensor picks up less of those harmful gases and thinks the cat is still installed and working fine.