OK, I have searched far and wide for the answer to this question and believe me, I cannot find a thread with all the needed info. I have a OBD0 B16a in a 91' Integra. Engine managment is controlled via a stock PR3 ECU. Now I know that my ECU is looking for 2 O2 sensors and that this has been discused many times but nobody seems to know where the corect sensor locations are. What I have found out in my recent forum searches is that one sensor reads the exhaust from the #1 and #4 ports and the other from #2 and #3. I guess my question is which sensor reads what? . I know that there is a primary and secondary sensor. Or does it even mater? Ive tried splicing into the one O2 sensor line, and I kinda figured it wouldnt work (which it doesnt like so many says it does). Another thing is I have a new header with a bung welded at the collector. If I put one of the sensors there it will give a reading for all four exaust gasses which to me would kind of contradict having the 2 in the first place. By now Im sure your confused as hell. I sure am. Well if anyone can answer this little 'riddle' it would be greatly appreciated. Thank you and have a good night

 

Primary O2 measures before the cat, secondary measures after the cat.

 

On the stock B16 header there are two O2 bungs. Both are before the collector let alone the cat

 

Yeah, for obd0 there are 2 O2 sensor like you said. one is on exhaust#1&#4 and the other one is on #2 and #3, both before collector. But the weird thing is those 2 sensor when came in to ECU is only 1 cable, is your the same? And I heard if you place the sensor the wrong way, you car performance will slightly poor.

 

There are two different Ecu pinouts on mine (PR3) for oxygen sensors. A primary and a secondary. I have tried splitting the one O2 sensor but all that does is give the computer two weak signals and makes it even worse. Im just not sure if its cool to switch them around (the #1 #4, #2#3 positions). Does the primary go in the #1/4 bung or the #2/3?

 

I'm not too sure which one is the primary, but I think both of them is important. I have no problem with mine cause I also use 4-2-1 header, so I can install it like the original.

 

I figured it out. Well actualy I emailed Hasport a while back and they just now got back to me. But for any future search references, the primary O2 sensor should read the #2/3 runners and the secondary should read the #1/4 runners. Thanks psx pat and '93 TurboD.

 

No problem,Bro. Just wondering did you also asked Hasport. What happen if we did it wrong the sensor between #2/3 with #1/4?

 

Is yours running rich? With both sensors tapped into one line on mine I was getting like 15 miles per gallon and it ran like ass. I know that the secondary should go to pin C8 on your PR3 ECU but Im not sure about the primary. I sent Hasport an email a while back and they wrote me back with that info. They said that the primary should go in the third runner and that the secondary should read in the fourth runner. Then again they also said if it runs funny, I might try switching them around. So all in all I guess using the bung on the collector that my header came with is out of the question. Ill just throw an old spark plug in there to plug it up.

 

Mine is running fine, but yes mine also running rich cause I just put in a new sparkplug and after I drove the car for 15 miles or so, the sparkplug became black on the ring. But again the car is running fine though. How many miles/ gallon your car can go?