where is the primary o2 sensor located?

[Flipmode]

i bout one at autozone but it has 4 wires and i found what ibelieved was the sensor to the right of the engine in the bottom. Am i looking at the right one? Also,the one i bought has 4 wires and the one i am looking at has 2. Am i looking at the right one???

 

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On the end of the header right before the Catalytic converter. Plugs in just above it. I dont think you will be able to access it from the engine bay.