Emissions systems

I hava a 2001 Nissan Pathfinder with a 3.5 V6 engine with 70000 miles. The service engine soon light come on. I scanded the system with a Auto Xray 6000 and it said the right bank was running lean. I took a reading on the oxygen ssensors and they didn't look to bad but I changeg them anyway. I changed the upstream sensor on the right side and the downstreem on the same side. While I was at it I replaced the downstream sensor on the left side, while I was down there. I didn't replace the upstream on the left sideas of yet. But it tested good at the time. I cleadred the codes and drove it down the road. About one mile down the road the light come on agaon. This time the code read that the upstream sensor on the left side had a high voltage output reading. The reading looked ok to me so I cleared the code again thinking it was a fluke when it cime on. I drove the car for about 30 miles when the light come on again. I scaned the system and it said the right bank was running lean again. What could be causing the system to trip with this code. Keeping in mind the oxygen sensors on that side are new and they are OEM sensors? I added injector cleaner to the gas tank when I was performing the work. Thinking that might help. What do you think?