Fxiong

hi club lexus people. i have come to a problem with my o2 sensor. i have CEL code 21,27,28. I change the tw0 main sensor that is located in the engine bay, but it still pop up the CEL code. so i decided to change the rear o2 sensor, but notice that the rear o2 sensor plug was unplug and didnt have a o2 sensor.So i went and buy a using rear o2 sensor for the car. Then i look under the car and it doesn't have a exhuast bung. So i decided to go to the muffler shop and weld a bung in so i can put the 02 sensor in. after all that i went to reset the car. So After i did all that stuff to the car it still pop up CEL 21,27,28. PLEASE someone help. I need to smog the car.