Genuine OEM BMW Rear Lambda Sensor, part number 11787589122. This sensor is installed in the exhaust system, typically after the catalytic converter, to measure the remaining oxygen content. This data helps the engine control unit make fine adjustments to the air-fuel ratio and assess the efficiency of the catalytic converter. A functioning rear lambda sensor is critical for emissions compliance and optimal engine performance. Genuine OEM parts are manufactured to exact specifications, ensuring precise calibration and reliable operation for emissions control.