DelSolDon

vehicle specs:
1998 Civic EX coupe
stock D16Y8
stock P2P-A84 ECU

So this code (P0135 or 41, depending on how you read it) showed up out of nowhere the other morning while on my way to work. I started by resetting the code and it will come right back about 3 seconds after starting the car. Checked to make sure the O2 sensor was plugged in all the way and it's fine. Checked the #15 fuse under the dash and it is fine. I had a backup O2 sensor and a backup ECU that I tried (one at a time) with no luck.

By going down the Helms flowchart, it suggests that I have an open or short in between pin A6 on my ECU and the O2 sensor plug. However, it doesn't really say which of the 4 wires it should be connected to. I had planned to just run another wire from A6 to the sensor plug, but am unsure which to connect to.

Can anybody shed some light on my problem? Thanks

SOLVED, SEE BELOW FOR FIX

DelSolDon

ok, so after looking at the wiring diagrams in the Helms, it turns out that A6 connects to the blk/wht wire on the sensor plug which is how i already had it connected. so now i am looking to replace the blk/yel wire and i have found that it leads back to the #15 under dash fuse and that is connected by the blk/wht wire in the C421 plug in the back of the fuse box.

so here is my next question, which of the 2 blk/wht wires in the C421 plug should i splice in to? does anybody have a pinout diagram for the C421 plug?

DelSolDon

well, i searched on here and found the pinout for C421 in this thread:
https://honda-tech.com/forums/showth...highlight=c421

thanks RonJ@HT

DelSolDon

ok, well i replaced both wires, the blk/wht to A6 on the ECU and blk/yel to connector C421 in the under dash fuse box. still getting code 41. i guess next step is to swap in the O2 sensor and ECU from my wife's '97 EX because i know those are good. will have to wait until tomorrow though, so any other ideas while i'm waiting?

DelSolDon

got home, tried with another O2 sensor and the CEL is gone. what i'm thinking is that when either the blk/wht or blk/yel wire got shorted somewhere, it caused the heater in the O2 sensor to fail. then when i swapped in another sensor without fixing the short, it caused that heater to fail... leaving me with 2 faulty O2 sensors.

FunctionovaForm

before replacing the 2 wires did you do any continuity test to check them, or did you just replace them?

DelSolDon

yes, i checked for continuity between the #3 and #4 pins on the connector terminals individually.