1996 Chevy Truck oxygen sensors

Hello, my check engine light came on so I had a buddy read the code with his scan tool. It came up as "po 160 o2 sensor no activity detected bank 2 sensor 2". The internet indicated that this was the o2 sensor on the passenger side after the catalytic converter, so I replaced it and cleared the code by disconnecting the battery for 20 minutes or so then let it sit without starting for 1/2 hour. When I went to drive, the light did go out at startup but came on again in about 2 km of driving. I read the code again and exactly the same message came up. I bought 2 sensors but do not want to replace the other one unless I'm sure because if it's not the problem I will take it back, they are expensive.