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Discussion Starter #1
I got and Obx header for my 95 240 and the lower o2 sensor bung is way to big in diameter. I know that putting this header on removes the cat that was originally between the 2 o2 sensors, which renders the second one pretty much useless. The problem with removing the sensor completely is that it is most likely going to throw an engine code, due to lack of resistance in the circuit. And using an adapter to put the sensor in will also trip a code, since it will sense that the cat is gone. I just would like to know what people have done to remedy this situation, preferably a cheap solution that does not involve cutting the exhaust after the second cat (in my case it is a real cat).
 

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sorry that i don't know how to answer your question. Instead I have a question for you. I just got obx headers for my 95 240sx also and i know what your talking about with the second o2 sensor and it not fitting. I found a pipe thread plug and stuffed it in there. My question is will my car run right without that second o2 being in there, i don't care if i have a check engine light or throw a code as long as it runs and drives right. I live in oregon so i dont have to worry about emisions either.
 

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I actually figured out that a 3 wire sensor from a G20 would fit in the lower bung and work with the plug on the 240. The only problem is that it's not reading the right oxygen level and making the car lean out like crazy. I did start my car with the sensor just hanging there (not in the header) and it seemed to run well without lighting up the check engine light; my guess is that it would probably run ok without the sensor there.
Let me know if you try it.
 

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I had Obx header also on my 95se and same problem with the bottom sensor fitment. So I just left the sensor out and putted a Bolt to seal it from leaking air. Check engine lights on but still ran normal.
 

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your my motha f'ing hero!
yay thank you. I have open headers as of the moment until i get my exhaust made tomorrow morning so i couldn't tell over the v8 nascar sound if it was running right. thank you!!!

oh and in regards to your delema I say get a pipe thread fitting plug that fits drill and tap it. tada!
 

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Discussion Starter #8
I had Obx header also on my 95se and same problem with the bottom sensor fitment. So I just left the sensor out and putted a Bolt to seal it from leaking air. Check engine lights on but still ran normal.
By left sensor out you mean unplugged and took off or just left it hanging there? And did it run better than with the stock header with the sensor off?
 

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I unplugged the sensor so it won't be hanging and just sealed it with a bolt. Um i say it ran the same but a bit differnece with the new header. (torque) But soon need to smog so time to take off.

Since the lower 02 sensor bung is to big in diameter, I've seen some people weld a Nut (that will fit the 02 sensor wire)over the bung to replace it. To make it work. Or maybe there's an adapter that you connect to it.
 

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The 2nd O2 sensor doesn't really do anything as far as engine performance, it just tells the ECU that the CAT is working...
Are you sure about that?

My results seem to say otherwise...

My solution was to put on a sensor off another Nissan that was the proper 18mm diameter and had three wires. After i put it on the car ran very lean and had awful power and throttle response; I gathered this was because the sensor was reading a lower o2 level due to the missing cat. So i put in a "spacer" between the sensor and the bung (made one out of a spark plug non-fouler), that caused my engine to run stoich or very close to it, which at least returned the horsepower of which I was being robbed and perhaps even more. The fact that spacing the sensor made my engine run stoich instead of lean causes me to believe that the downstream o2 sensor somehow affects the amount of fule being dumped into the engine.

Correct me if i'm wrong or explain why my situation may have happened.
 

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correct they do make reducer bushings for all sizes that should take care of the fitment prob and if you cant find one just hack off the big one and get a o2 bung from a exhaust shopand weld it in its place
all o2 sensors talk to the ecu and will do more then tell that the cat is working fine it will cause you to run lean or rich if its not right even if you are a n/a car it will still ned some back pessure as well also u can burn up your head dew to a bad o2 .but thats just what ive learned about o2 sensors
 
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