Sensor 2 of the Big Dipper oxygen sensor assembly 1 is a rear oxygen sensor. Sensor 2 of cylinder bank 1 is the rear oxygen sensor facing the left cylinder of the engine. Sensor is a kind of detection device that can sense the measured information, and can convert the sensed information into electrical signals or other required forms of information output according to certain rules to meet the requirements of information transmission, processing, storage, display, recording and control.