Diagram of the location of a down stream o2 sensor on a 2005 chevy malibu.

Question edited for clarity.
Question moved to model category.

Yes there are three o2 sensors

Bank 1 in the exhaust manifold
Bank 2 in the exhaust manifold
One after the Catalytic Converter.

Not sure what you mean 'where is the best place to put it'? The scan tool tells you which one needs replacing by a P**** Code, or are you just replacing one for fun?

Answered here on JustAnswer dot com

The o2 sensors are located in the exhaust pipe. The one you are looking for is the post cat sensor. The sensor furthest back in the pipe after the catalytic converter.
Mar 19, 2007

Chevy S10 Oxygen Sensor Location: Q&A Guide - JustAnswer

`look under vehicle at exhaust the tube looking part with a connector thats after the cat converter going towards tailpipe is down stream o2 sensor

EPS # 41 80 amp , located in the under hood fuse box . Plus EPS fuse 2 amp , located in the BCM fuse block .

http://www.autofuseboxdiagram.com/1926-2005-chevrolet-malibu-ls-fuse-box-diagram

#1 is the up-stream sensor (The O2 sensor located before the Catalytic Convertor). #2 is the down-stream O2 sensor. This sensor is located after the Catalytic Convertor. I hope this helps, let me know if you have other questions concerning this issue. Good luck.

Here is a video of the removal of the front seats of a Chevy Malibu that is in the same series as your 2005 (this video is applicable for 2004 - 2012 model Chevy Malibu)
https://www.youtube.com/watch?v=2l5xLKNRa2g

BANK 1 UP STREAM OXYGEN SENSOR LOCATION IS LOCATED ON THE REAR EXHAUST MANIFOLD.

I believe you may have three (3) O2 sensor, (2) of the sensor is on ether side of the exhaust down pipe and some times called up stream O2 sensor, the third O2 sensor know to some as the down stream O2 and is after the Catalytic converter.
The up stream O2 monitors the left and right side banks and sends signals to the ECM to make the adjustments to the injectors to keep the proper air/fuel ratio.
The down stream O2 monitors the catalytic converter and sends signal to the ECM if the catalytic converter has gone bad.
Good luck and hope this helps. If you get it scanned at Auto Zone, they should be able to let you know which O2 has gone bad so as to not change all three. You could always check the wire harness to the down stream O2 for corrosion.

I have no diagram,but there is what they call upstream,and down stream locations.Upstream,is above the cat-converter,the down stream is below the cat-converter.This would be per-side.You have a left,and a right exhaust manifold.The exhaust pipes run from each manifold(left and right)to the catalytic converters.So you would have to have on the left side a up stream o2 sensor,above the catalytic converter.and a down stream o2 sensor,below the catalytic converter.this would also be on the right side also,this would be the total of four(4) o2 sensors.I hope this is helpful.

Under the truck, driver side exhaust pipe, there are 2, 1 is before the catalytic converter, the other is after. They screw into the side of the exhaust pipe.