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Thread: BMW 330i p0420 precat oxygen sensors

  1. #1
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    BMW 330i p0420 precat oxygen sensors

    I apologize if I’m asking a dumb question I have been trying to figure out the reason behind my p0420 and p0430 codes. I looked at the precat oxygen sensor voltages and I’m seeing both sensors showing 1v and remains constant. It fluctuates slightly with throttle liftoff. MAF is showing normal readings, and I haven’t been able to find any vacuum leaks. The post cat oxygen sensor is fluctuating like the precat sensor should be. Does anyone have any ideas on why I’m getting these readings?

  2. #2
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    BMW 330i p0420 precat oxygen sensors

    Last edited by RobertDenali; 06-13-2019 at 03:44 PM.

  3. #3
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    01 E46 330i 5sp man spor
    Maybe the o2 sensors are plugged into the wrong connectors.

    Sent from my S61 using Tapatalk

  4. #4
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    I understand that they should alternate, but the problem is that they don’t. It seems like the precat and post cat sensors were switched around with the readings that I’m getting. That is why I’m confused on what the problem is.

    I took a look at the o2 sensor plugs and there’s really only one way they can be plugged in. A little while back I got a code saying the post cat sensors were swapped so I tried swapping them around and I got the code again. After swapping them back the code went away.

    Would the problem be cats? I want to keep that as my last resort but if it seems to be pointing towards bad cats, I’ll have to replace them.

  5. #5
    Join Date
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    BMW 330i p0420 precat oxygen sensors

    Well typically if the voltage doesn’t change and the O2s are good to include being wired correctly then I would suspect the cats.

  6. #6
    Join Date
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    2003 328i
    Those codes are catalyst efficiency. This is detected by the post cat sensors when the converter efficiency drops below a minimum threshold.

    If the post cat sensor is switching with the engine warmed up, there is no misfiring and the fuel trims look good, the cats are not working.

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