KA-T exaust, what do you guys do with the 2nd O2?

[node]

Im fabricating my downpipe and exaust system and im not sure what to do with the second O2 sensor on my 95 KA-T. Can I leave it dissconected or will that throw a code?

If it throws a code can I just use the output from the primary o2 and a resistor to fool the ECU? Any info on what you guys do is helpfull. I really dont want to weld in a bung for a second sensor.

[Checkered-Member]

Im fabricating my downpipe and exaust system and im not sure what to do with the second O2 sensor on my 95 KA-T. Can I leave it dissconected or will that throw a code?

If it throws a code can I just use the output from the primary o2 and a resistor to fool the ECU? Any info on what you guys do is helpfull. I really dont want to weld in a bung for a second sensor.

Throw a code? YesUse output from the primary O2? that might work...Solution? I know one, get a cat (save the planet) and put the O2 sensor after it.

[node]

Anyone with any usefull information?

I cant beleive that all of you fellas have two O2 sensors in there.

[klattr1]

i left mine hangin.Yes it will throw a code but its just for diagnostic purposes only. It basically tells you if your cat is bad.I just left mine out but im lookin for a resistor to put inline so the dang code doesnt come up each time.

[node]

Oh so the check engine lite stays out? Cool.

As far as the resistor goes I think it will need to be of a very small value. Also since the sensor is a three wire we might need to trick the computer into thinking the heater circut is still there. I guess a ceramic resistor in place of the heater circut would do that.

[klattr1]

im not sure exactly which one to use but I would like to know also if you find out what resistance to use.ThanksI prolly could call Jim Wolf and they would know.

[bumpermatic]

Would an O2 Simulator work for this situation? I've herad of the Spec V guys using this to deal with aftermarket headers they get.

http://www.highrevmotorsports....d=122

or google for O2 Simulator

[daniel240]

i just unplugged mine. and it threw a code. you can have it erased though. i think it stays erased for like 1000 starts or something like that. autozone wont do it anymore, i get mine erased at the nissan dealer i work at. they would probably do it if you tell themyou have no 2nd cat. but ive never had a 2nd cat even before i had a turbo, i just disconnected it when i got my turbo and all new 3'' exhaust.

[AceInhole]

it stays erased for about 20 seconds, depending on how rich you like to run.

otherwise just shove the 2nd O2 in there like a man.

[j-z]

yeah it seems like alot less hassle to weld a bung than to try and figure out what goes where and how this does that.

[C-Kwik]

My downpipe came with a fitting for it. I have no catalytic converter and it doesn't throw a code at all. I'd say weld in a bung and keep it in the exhaust stream as usual.

[chandler]

the second O2 sensor is for cat efficiency, thats all it does, it throws a code when its sees a voltage near or exact to the first o2 sensor, get a code blocking insert or simulator, that will fool it into thinking its still got a working cat

[node]

Based on what chandler says, we could just take the signal from the primary O2 and split it to the secondary O2 signal wire through a small resistor. That would simulate less oxygen in the air right? Because less O2 = less voltage.

I think the heater circut also needs to be fooled but that should be easy to do. Man I need to go buy a new multimeter!

[C-Kwik]

And again, I do not get a code from mine at all from the second O2. I have no cat and no EGR. I'm thinking as long as it receives some kind of a reasonable signal, it's fine. Keep in mind that this is an O2 sensor, not a multi-gas analyzer. As long as the oxygen content is within acceptable ranges, it shouldn't trigger a code. Leaving the sensor hangin would likely throw a code since it will probably detect too much oxygen, and/or it might not see enough heat to effectively measure anything. Unplugging will obviously throw a code since the ECU won't be getting any feedback from it.

[Big Manne]

i thought 95 were odbI. so my 95 240 has 2 o2 sensers?

[node]

Well my 95 is not a california model or anything, so yea I guess they all do. The ECU is OBDII as far as I know. It has the newer wiring harness and the engine has the crappyer cams. Perhaps mine is jsut a later production date. When did they change over to the OBD2?

[Big Manne]

i thought they changed in 96

[chandler]

O2 sensors act as little batterys, generally they produce between .01 and .90 of a volt, they use zirconiadioxide as a catalyst, the 21% o2 in the outside air, and what ever is in the tail pipe causes voltage to be generated. a resistor in the second o2 wiring would not help , they make plug in type "replacement" housings that just send a false reading. the second o2sensor is just to moniter the cat, it should read between .00 and .05 of a volt. any more than that and it should set a code. obdII was manditory compliant in 96 but most import manufacturers voluntarylly(spell) complied to it soon than that

[kenji_m]

I have a '95 as well. My rear O2 sensor went bad and threw a code. Eventually I got tired of the check engine light so I just wired in an 8 ohm 20 watt resistor (Radio Shack Catalog #: 271-120) in place of the rear sensor. It's been a few thousand miles now and no more check engine light. The 20 watt power rating is very important, you'll just melt the resistor if you don't use a 20 watt one.

[klattr1]

thanks alot for that info!so did u install it where the wire goes into the ecu or did u literally install it after the sensor?

[kenji_m]

Sorry, I should have been specific. I installed the resistor where the old O2 sensor was. I cut off my old O2 sensor and kept it plugged into the harness. If my memory serves me correctly there are 3 wires going into the sensor. A power, ground and signal. I believe the power wire and signal wire are on the outside of the harness and the ground wire is in the middle. I wired the 8 ohm resistor to the power and signal and I grounded the ground wire to a metal surface on the chassis. Then I secured the resistor to the chassis so it's not flopping around. That's all there is to it.

[chandler]

that makes sense, I didnt understand what you mean by resistor, but on later models it might set a code for heater circuit in op wont it?

[jtsarnak]

Punch me if I'm wrong, but if you "trick" the computer into thinking the sensor is there, how does it adjust the air/fuel ratio or whatever is controlled by the O2 sensor's input data? Would this change the way the car manages itself?

[kenji_m]

This only applies to the rear oxygen sensor on 95+ 240sx's. It's there to make sure the catalytic converter is doing it's job. I believe on the OBDII cars the computer looks for a varying signal from the rear oxygen sensor so that's where that o2 simulator that bumpermatic posted would come into play. I don't know if the resistor trick would work with the newer cars because it only sends a fixed signal, that's within spec, to the computer.

[Camel]

just weld an o2 bung behind the brick on the cat. works like a champ. the rear o2 and front o2 are looked at together by the computer and if the readings are out side of tolerances, it pops a code.

i welded an o2 bung to my random tech and it solved my check engine light problem. no big deal.

[mean green s14]

a nissan tech told me on the 95 and later 240s the second 02 sensor just lets the computer know if the cat is working or needs to be replaced, this is done by the second 02 sensor having a different reading than the first, if you simply unplug it the car will run no different at all. it will just have a engine light to tell you to get a new cat, but you dont have one so who gives a crap, just ignore the light!put a piece of electric tape over your gauge class and youll forget its on

[jtsarnak]

Well, my light has come on a few times previously, and everytime it has, I have used the self diagnostic tool on the '95 ECU to read the code (as per the FSM). Everytime the light has come on, it throws TWO codes, one for the oxygen sensor and one for the knock sensor. Neither have ever shown up by themselves and I hear no knock at all when the light is off or once it's been tripped and driving it around with the CEL on. This leads me to believe that the readings from the second O2 sensor are giving the ECU instructions on how to adjust the air/fuel mixture in relation to it's readings, hence the temporary knock issue when the O2 code is thrown. Any thoughts on this? Is it just the brief change in readins that confuses the ECU and causes the knock sensor to tip or is it the case that if I am driving on a regular basis with the CEL light on for the O2 sensor code, my car is running on retarded timing due to the codes being thrown?

[klattr1]

well,i have no rear 02 sensor and I'm getting the knock sensor code all the time. They both are instant and I think my timing is being retarded due to my lower hp numbers(310 rwhp at 17 psi on t3/t04e).I drove my car and both codes come back after like 5 min.I expect the rear 02 sensor code to come since its not even there but I wouldnt have thought that it could be tied into the knock sensor.My first plan is to just replace the knock sensor and see if the knock sensor code comes back.I got to make more power than I have. I am determined.