Oxygen Sensor Replacement

I have a 200l LS V-6. I am trying to guess at the problem with the
"Service Engine Light-Fuel Trim System Lean" I changed the oxygen sensors
on both manifolds. It was suggested at the site I bought the sensors from,
that I change all the sensors.Is anyone familiar with how many sensors
there are and where they are.

 

There are 4 sensors. Bank 1 sensor 1 is closest to the firewall on the top, before the cat converter. Bank 1 sensor 2 is straight down from it, below the converter. Bank 2 sensor 1 is right behind the radiator on top, before the converter, bank 2 sensor 2 is right below the converter and can be seen and accessed from below the car. If you raise the car and support it properly, follow the front exhaust pipe down and you'll see it right below the A/C compressor. There is enough space between the cradle or frame rail and the exhaust pipe to get an open end wrench in and reach it. As someone who has done mechanical work for about 50 years, I was always told and taught that the sensors should be replaced in pairs, both uppers or both lowers, but never heard that they should all be replaced at once. Let me correct myself here. I've heard and been told that by parts sellers, but never from any factory service rep, service guide, or mechanic. When you replace your sensors, be sure to torque them to the proper amount of pressure or they may leak air in around them and create a problem.