ok i had to change one of the stock senors on bank 2(downstream) cause of low voltage, when i did i cleared the codes and then went driven around then in 30min the check engine light comes back on and get it check and it says that bank 2 upstream O2 senors is running lean. any ideas

 

You should drive your car around with the scanner that you brought the code up with. This way, you will be able to see what all of your O2 sensors are doing. If you see one that stays at a certain value for a pretty long time, especially after you step on the accelerator, you have a lazy O2 sensor than. An upstream O2 and a downstream O2 are two different O2 sensors. The upstream is closer to the motor along the exhaust system, the downstream is closer to the cat along the exhaust system.