which O2 sensor is it?

[tacoboy]

97 Strat 2.0 134k. Almost since I have owned this car for the past 3 years, the check engine light has been mostly on. When checking the engine codes, "21" comes up as the oxygen sensor. Trouble is, I hear there are two of these babies. How do I know which one it is? Is there a voltage measurement I can take? I am about to look this up in my Haynes manual.

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[Brock0295]

the first o2 sesor tells the engine to go ritch ot clean it is right on the manifold. all the second one does is tells the engine if the cat converter is working right. if there is no hesitationg or sputering when u take off... i would replace the upstream one thats on the manifold.

[tacoboy]

Thanks for the info. I was looking in the Haynes manual the other night and it references the two sensors but only pictures the downstream one after the cat. What I think the upstream one is does not resemble what they have pictured for the downstream sensor. It is sort of an oblong thing that is in the exhaust manifold pipe with 4 wires going to it. Haynes doesnt't detail this and I get 5V constant on 2 wires when the ignition is on (engine not running). Is there some test I can perform on this sensor to see if it is working properly?