Check engine light on

Hi every one, my name is Alan from (Honolulu). I have an SUV my engine light came on two weeks ago, then take dealership services , the dealership check out is problem,(bank 2 sensor 1 oxygen sensor) is problem,too rich and lean. I have change new oxygen sensor, am say good find out the problem, just go for it to change new oxygen sensor. after five days of driving ( engine light back on again), then take back dealership ask why my engine light back on again, after they check same problem again (bank 2 sensor 1 oxygen sensor ). they known change new one two weeks ago. also, they said it will turn off by itself.