There's a heating element in your O2 sensor. While changing the down pipe it may have gotten banged and messed up the element. When the shop did mine they hit it putting it in the new downpipe. Had to buy a new one because of it.
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2017 BMW M240i xDrive - Mission Performance Stage 2 and transmission flash, CSF Intercooler, VRSF 4.5" downpipe, Dinan midpipe, BMW M Performance muffler, Agency Power Cold Air Intake. 11.58 @ 116MPH
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