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Ignore cat code P0420 until you fix the upstream O2 sensor issue, which seems to be caused by an open sensor circuit. Make sure that the O2 sensor connector is full seated and that the connector pins are not corroded or damaged. Next inspect all of the O2 sensor wires for a break or damage. If you find no issues, replace the O2 sensor.
Ignore cat code P0420 until you fix the upstream O2 sensor issue, which seems to be caused by an open sensor circuit. Make sure that the O2 sensor connector is full seated and that the connector pins are not corroded or damaged. Next inspect all of the O2 sensor wires for a break or damage. If you find no issues, replace the O2 sensor.
Thanks again! I will check that out! Happy to have this dude running better. Very happy with the idea to buy that better Creader Elite! It went on a lightning deal on Amazon for $118 so I snagged it up. Much better than the old ELM BT reader I was using before.
Ignore cat code P0420 until you fix the upstream O2 sensor issue, which seems to be caused by an open sensor circuit. Make sure that the O2 sensor connector is full seated and that the connector pins are not corroded or damaged. Next inspect all of the O2 sensor wires for a break or damage. If you find no issues, replace the O2 sensor.
Is the upstream a 4 or 5 wire? I keep seeing both options.
I replaced the O2 sensor. The old one was toasty looking. New one in, car runs smooth again. Smooth idle. However, getting "oxygen sensor heater before catalytic converter activation" error. Not sure where to go from there.
If you just replaced the sensor, did you reset the codes after? If not, reset the codes and see what comes back. (if anything)
Yeah, I reset the codes, cycled the car ignition like normal, and the code comes back. I took it for a test drive and still there. I have driven it about 100 miles since swapping. Car runs smooth and no issues, just the code. Definitely fixed the rough idle. One thing I notice, I am getting like 32/33 mpg and not the 38/40 I was before.
Yes, I only replaced the upstream as that was the only code I was getting before. Should I replace the second one as well?
Is the current upstream code identical to the original upstream code? If so, check whether the connector is fully seated and whether any pins are bent or damaged. If those are fine, I suggest that you voltage test the heater power wire at the unplugged sensor connector and continuity test the heater ground wire between the DME and sensor connectors.
Is the current upstream code identical to the original upstream code? If so, check whether the connector is fully seated and whether any pins are bent or damaged. If those are fine, I suggest that you voltage test the heater power wire at the unplugged sensor connector and continuity test the heater ground wire between the DME and sensor connectors.
Same codes as before. I will go out and check the connector. I made sure everything was straight and no corrosion before plugging it in. I will double check though.
Which wires are you referring to? I do not have a wiring diagram for this beast. If there is a good one out there you can point me to, that would be greatly appreciated!
The code actually suggests that the problem lies on the DME activation/ground side (pin 3, white sensor wire) of the upstream sensor heater element, so start there.
For the continuity test, the wire diagram further suggests that the heater element white (ground) wire runs to pin 30 of DME connector B (middle 53-pin connector X60231 for the N12).
If you don't read continuity between sensor pin 3 and DME pin B30, then repeat the same test between sensor pin 3 and DME pin B41. Also, on your door jamb sticker, what is your car's build date?
The test can be done safely if you disconnect the battery, the sensor connector, and DME connector B.
Last edited by Maybe, maybe not; Apr 10, 2026 at 04:13 PM.
If you don't read continuity between sensor pin 3 and DME pin B30, then repeat the same test between sensor pin 3 and DME pin B41. Also, on your door jamb sticker, what is your car's build date?
The test can be done safely if you disconnect the battery, the sensor connector, and DME connector B.
Your 09/09 build date is in the LCI-Transition window. For your build date, the upstream O2 sensor heater control (PWM) is potentially located at DME pin B11. Check whether DME pin B11 is a white wire or possibly a white/yellow or white/green wire.
