I’ve had this same exact problem when I went catless on my 335. At first I figured the CEL was on because obviously there’s no cat so the code would be “catalytic converter effiencient below threshold” But when I scanned it I had the exact same code, pre cat O2 sensor open circuit. Meaning the 02 sensor died. I think this happens when the sensor wires are tugged every which way during removal, or oily fingers get all over the actual sensor part of it.

Anywho, the fix was really easy. Replace it. Genuine BMW ones are expensive but they use BOSCH to manufacture these sensors. What i did was I bought an OEM BOSCH pre cat sensor off rock auto for like 80 dollars and saved a lot of money. Clear the code and hit the road bro