BMW S55 - Post-Cat Oxygen Sensor - 11787848486Located after the catalytic converter on the longer downpipe. Product Details If your engine is tossing you an oxygen sensor check engine light, replace it with this genuine BMW unit. The O2 sensor measures the amount of unburned fuel in the exhaust pipe to adjust fuel mapping for less emissions and better fuel economy. When malfunctioning, your MPG and performance can suffer. Replaced fairly easily, you can restore your performance and mileage