OBD2 Secondary 02 Sensor Wiring

When performing a B-series engine swap into an OBDa/b Civic hatchback and retaining a U.S. OBD2 ecu is necessary, swappers will need to wire in the 2nd 02 sensor in order to please the ecu.

This is the case for people who are running an EX or Si harness in an EK hatchback, coupe, or sedan for their swap.  The EX and Si engine harnesses do not have the 2nd 02 sensor integrated into the engine harness, but rather into the cabin harness of the EX & Si chassis’s. Whereas, the the non-VTEC engine harness has the 2nd 02 sensor integrated into the engine harness itself.

All that is required is to extend the 2nd 02 wiring to reach the 2nd 02 sensor when using a non-VTEC harness.  Not such the case for Si & EX harness users; they will manually have to wire in the 2nd 02 sensor directly to the respective ecu connectors.

Here’s the pin locations:

2nd 02 Sensor ECU Connector
White Wire    TO D14 – Sensor Heater 02 Signal (SH02S)
Green Wire  TO D11 – Sensor Ground 2 (SG2)
Black Wire  TO A5 – Secondary Heater 02 Control (S02HTC)
Black Wire  TO A11 – Power Source 1 (1GP1)
2nd 02 Sensor ECU Connector
White Wire  TO A23 – Sensor Heater 02 Signal (SH02S)
Green Wire    TO C18 – Sensor Ground 2 (SG2)
Black Wire  TO A8 – Secondary Heater 02 Control (S02HTC)
Black Wire  TO B1 – Power Source 1 (1GP1)

26 Replies to “OBD2 Secondary 02 Sensor Wiring

  1. Katman,
    I can’t find the Primary o2 sensor on my JDM B18C OBD1. I can see the secondary on the down pipe but I can’t find the primary. Is it because there is not 2 but only 1o2 sensor on this engines?

    Car Spec.
    car: 97 – EK9/OBD2
    engine type: JDM B18C OBD1


  2. Leo,
    That is correct, OBD1 engine’s (and all JDM OBD2 B-series engines for that matter) only use one 02 sensor and its usually mounted on the down pipe near the collector.

  3. Katman,
    Thank for your help!
    One more question, is there anything i need to do or can do to avoid that cel?

  4. I have a OBD1 JDM GSR complete stock engine w/ the 98 spec type R headers. The primary O2 sensor is killing me with the wiring! I have a OBD1 Stock JDM ECU with a jumper hardness (OBD 1 to OBD2A) My Engine Wire Hardness is a OBD2A (it had the V-tec wires already in it) I bought a new universal O2 sensor from autozone and that O2 sensor has four wires just like the one that came with the engine. The autozone O2 sensor has one black wire (signal), two white wires (heater), and one grey (ground) My old O2 sensor has one green, two black, and one white. My check engine light blinks only one time and keeps repeating. I can not get that check engine light to turn off. Side note: i have a second O2 sensor after the cat (not one the cat, after it) it is in place but not wired. The wires I am using from my engine wire hardness are: black w/ yellow, black w/ white, solid white, and another wire that is surrounded with a braided wire coated grey ( actual wire color I forgot at the time) Any help is appreciated and good work on the website.

  5. Ruben,
    What year is your vehicle? I’m assuming OBD2 as you mentioned you have a 2nd 02 sensor.
    Need to know before helping you any further.

  6. Leo,
    You can either trace the 02 wiring back to the primary 02 ecu pin locations (referencing the information in the article above), or run a chipped OBD1 ecu with disabled 02 sensor.

  7. great write up. I spent a few hours shuffling through garbage and dead-ends before coming across your excellent write up.
    With my 97′ civic EX i plan to swap in a JDM 98spec+ B18c w/ a p73 ecu and reuse my EX harness.
    – Am i correct in understanding that i wont have to worry about the CFK issue?
    -will the P73 ecu plug right into my engine harness.

  8. Stu,
    Thanks! Because you’re using a JDM OBD2 ITR ecu, the oil pump mounted CKF sensor is sans from the programming (as it is on OBD1 ecus’); 2nd 02 sensor is sans as well.
    Yes the ITR ecu will PnP to your 97 ‘EX being that it’s an OBD2a ecu and your civic is OBD2a spec too.

  9. So the secondary o2 harness inside the cabin is completely useless (SI / EX). You have to hard-wire it to the specific ecu connectors to get it to work no matter what if using a ecu that is looking for it?

  10. Stu,
    It’s not useless if you’re using an EX/Si engine + cabin harness. One you mix it up and use an Si/EX harness in any other civic model besides an EX/Si, is when you have to hardwire the 2nd 02 (if sporting a US OBD2 ecu). All other 96-00 civics (CX/DX/LX) all have the 2nd 02 sensor wiring integrated into the engine wire harness.

  11. So I’m using a obd2a ex harness for my d16y8 into my ek hatch. Can I just tapped into the ecu harness by the ecu or should I cut them and connect the o2 sensor to the wires I cut?

  12. I have a 98 DX converted to an D16Y8 M/T (OBD2) VTEC with only one O2 sensor wire set. My understanding is that I have to add a secondary O2 sensor to get the check engine light off. Currently the headers are holding the primary O2 sensor using the wire harness. But the secondary O2 sensor is disconected inserted at the catalic converter with no wires arround to connect them. As you posted, I should follow this pin out.

    2nd 02 Sensor ECU Connector
    White Wire TO D14 – Sensor Heater 02 Signal (SH02S)
    Green Wire TO D11 – Sensor Ground 2 (SG2)
    Black Wire TO A5 – Secondary Heater 02 Control (S02HTC)
    Black Wire TO A11 – Power Source 1 (1GP1)
    You recomend I cut and splice the cable at the connector or should I just depin it and re pin the new 4 wire set for the O2 Sensor. Sorry for the long post but I wanna get everything down before attemping it.
    Thanks in advance!

  13. hey katman,
    just wondering if the 2 black wires are interchangeable?
    likw does it matter which 1 of the black wires go to a8 or b1???

  14. Katman, I’m swappig a jdm b16a obd2 engine into my 98 ex civic.. I can’t use my jdm ecu? If so what ecu should I buy?

  15. Justin- I’m not sure what your goals are (if you want to BAR your car or just get the car running) but you most definitely CAN use a JDM B16A ecu in your 98 EX if its an OBD2a ecu. It should plug right into your harness. If you want to be able to BAR your car, then no, you cannot use a JDM OBD2 ecu because JDM OBD2 ecu’s cannot communicate with a US OBD2 scanner tool.

  16. no i dont need to bar, i just got a little confused. like my earlier post im going to swap obd2a b16 into my obd2a 98 ex.. what will i have to do in order for the motor to run correctly.. is it just extend o2 sensor? do i have to do the bypass aswell? and do i have to do the IACV fix aswell? or is all this done because i have a ex 5 speed wiring harness?

  17. Hey I have an obd2 wiring harness with the obd2 to obd1 adapter and my o2 sensor is sending a cel and My harness goes from a 7 wire to a 5 wire but my obd1 motors o2 sensor is a 4 wire what can I do Katman. 1997 honda civic hx with B16a jdm

  18. JC- Depending on what actual engine wire harness you are using, this will dictate what you need to do. If you are using a 96-98 CX/DX harness, the 2nd 02 wiring is already there (pre-wired from the factory), you just need to extend the 2nd 02 wiring to reach the 2nd 02 sensor. Now, if you’re using an EX harness, there is no 2nd 02 wiring embedded in this harness like a CX/DX harness, though, the 2nd 02 wiring is populated on the EX harness (weird i know). So, you just tap into those 2nd wires at the ecu plugs and extend wiring out to the actual 2nd 02 sensor.

  19. :mrgreen: Im using the wiring harness from a obd2 motor that was in was a D16y5 sohc I have a obd1 ecu with the adapter harness on it my harness goes from a seven wire into a plug in piece then its 5 wires and my b16a is a 4 wire o2 sensor.
    One question if the o2 sensor dont work it wont let your vtech work cause my vtech isnt working ❓

  20. Im buying a chipped ecu from phearble for 175 and i dont have to worry cuz theyll delete th o2 sensnor

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