Can't get the engine light to stay off after replacing the 02 sensor downstream

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I have a 2006 nissan altima 2.5s. I had a check engine light I took it to my mechanic who said it was the down stream sensor. The code that came up was bank 1 sensor 2 heater circuit low. So he changed it cleared the code. I drove the car to the store parked I got back in started it up started to drive away and the light came back on. People on the nissan boards had said if I replace the upstream sensor it would fix the problem. The downstream was replaced in April and the upstream a couple of days ago. Now the codes were erased I drove to work about 20 minutes away again shut the car off started it up and the light came on again I took it to pep boys were I know the service manager and explain ed it to him. He checked it and said the downstream 02 sensor was bad. So he changed it. Rest the codes I drove the car for about 30 minutes or so. again parked go out Got back in started to drive and the light came back on. I go back to pep boys but the service manager left for the day I explained it to the mechanic who worked on the car and he said the downstream 02 sensor is bad. As I drove away I got maybe 5 or 6 blocks 2 out of the 3 monitors were set. I was told that I could pass inspection in nys . But at the same ti.e the check engine light was on. I'm at my wits end with this car. Can anyone help