I have a 96 gt auto, and I need to replace the bank 2 sensor 1 oxygen sensor. Guy at autozone told me it was the upstream drivers side, but i really don't trust him. I haven't found a diagram online, so if anyone has one or knows which sensor it is I would greatly appreciate it.
The autozone guy is correct. Bank one is the pass side. (so you have a driver side issue since its bank two) and sensor one is pre cat, sensor two is post cat (or down stream by trans tail shaft if you've removed the cats).
Your opening post says, "I need to replace bank 2 sensor 1 oxygen sensor". As squeak93 as stated, bank 2 is the driver's side. Sensor 1 is upstream.
The DTC calling out "system efficiency below threshold bank two" is the DOWNSTREAM O2 sensor. This DTC could never apply to the upstream O2 sensor.
HOWEVER, in the vast majority of cases the DTC calling out the downstream O2 sensors does NOT indicate a problem with the down stream O2 sensor. The problem is with the cats or the sensors supporting the motor's operation.
IMO if you insist on replacing an O2 sensor as a trouble shooting step, your $$ is better spent on the upstream O2 sensor as it will actually affect the motor's operation. If the DTC continues THEN replace the down stream O2 sensor with the sensor that was removed earlier. This gives you a "two for one" trouble shooting bang for the buck.
There's a saying that goes, "cats never die. They are murdered". What that means is a cat converter rarely go bad on their own. The are killed by conditions external to the cats. For example"
Excessive oil in the exhaust
antifreeze in the exhaust
mixture too rich
mixture too lean
exhaust too hot perhaps from a non working EGR system.