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The purpose of this write-up is to help people with SR figure out which O2 sensor to use. This question comes up at least once a week. I hope this will answer all your questions concerning fitment and operation of your oxygen (O2) sensor.
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Search words:

oxygen o2 ho2s sensor heated fuel economy gas mileage rich lean exhaust narrowband wideband turbo elbow extension housing megan greddy
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References cited:

1. o2 sensor
S13 SR20DET REDTOP: 22690-19P20 ('87 280ZX)
S13 SR20DET BLACKTOP: 95 300ZX TT
http://www.zilvia.net/f/showpost.php...89&postcount=4

2. A lot of great information in this thread.
http://www.zilvia.net/f/showthread.php?t=65359

3. Pictures from me.
http://www.zilvia.net/f/showthread.php?t=103302
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Terms:

Redtop = early 90s S13 SR engines
Non-redtop = everything else, including blacktop S13 SR, S14 and S15 SR
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SR O2 sensors come from the factory in 2 different sizes:

* Redtop - "skinny" on left in the picture
* Non-redtop - "fatty" on right in the picture



When choosing an O2 sensor, there are 3 things you need to look for:

1. Does it fit in the o2 sensor housing? It has to physically thread in. Luckily sensor sizes are standard and it's easy to fit.

2. Does it have the correct number of wires? Sensors come with between 1 and 5 wires, depending on whether they have a heating element or not, or if they're wideband/narrowband. The KA uses the 1-wire unheated narrowband unit. The SR uses the 3-wire heated narrowband unit.

3. Does it have the correct harness plug and are the wires long enough? This isn't a big deal because you can repin it and/or extend wires as needed, but getting the right size will save you a lot of time. It should be plug-and-play.
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How o2 sensors work:

There is an ideal air-fuel mixture for your exhaust. The purpose of the o2 sensor is to detect your actual exhaust composition and report that information to the ECU so that air and fuel ratio can be adjusted.

Narrowband sensors are really simple. They can only tell you if you're above or below the ideal level. They don't tell you how far off you are.

Wideband are more complicated because they tell you how far off you are in addition to being above or below the level.

For more in-depth explanations, search or look in link#2 above.
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Stock wiring:

The stock SR wiring has 3 pins for the o2 sensor. The middle wire is for signal. The 2 outside wires are for non-polar 12V power for the heating element. It doesn't matter which 12V pin is (+) or (-), as long as you hook both up. Current will flow through one way or the other way and the heating element will work just fine.

I have confirmed this on redtop the. Check #1 in my sig for S14 SR FSM - EC section. It should be the same as well.
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Confirmed fitments:

You have to first determine which o2 sensor housing you have. If you have a stock housing, obviously it will only fit what it came with from the factory. If you have an aftermarket unit, check to see which size it is. Look in link#3 above. Most aftermarket units will fit the fatty, and come with a reducer bolt to use a skinny.

You then have to look at your wiring to see if it's the same as described in the section above. If you're using a stock harness you should be fine.

Skinny:

a) Z31 (pre-90 300ZX) - this one should fit and plug in.
b) S12 (pre 90 200SX) - this one fits but the wires are too short. you have to extend the wires.
c) Get an OEM unit from a JDM vendor.

Fatty:

a) Z32 (90-96 300ZX and Twin Turbo) - this is the preferred unit since it fits, the wires are long and plug in, and it's easy to find.
b) B13 (91-93 Sentra SE-R) - this fits, wires fit, but the wires may be just a tad too short, depending upon your setup.
c) Get an OEM unit from a JDM vendor.
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Tips for removal and installation:

* It's best to work with the sensor when the engine is out because you have more room.

* The skinny socket is hard to find. Most places would have to special order it. 17mm wrench will fit if you can find room in the engine bay to stick your hand in. The fatty socket is easy to find.

* To help removal, snip off the wires on the old unit since you're throwing it away any way.

* A hot unit will actually be easier to remove since the metal housing expands slightly under heat. Wear gloves! (I have scars because I was being tough )

* Spray some thread penetrant and let it soak overnight.

* Use high-heat antiseize compound on the threads.

* There is no prescribed torque value, but it should be torque to around 30-35 ft-lb after the gasket contacts the housing.
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I hope this article was thorough enough to answer all of your questions. If you have any more, please feel free to post up or contact me directly.

Whoever said Zilvia never did anything for you?