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