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    Mercedes C200 Kompressor (WDB203045) 2001 - MIL on, codes P0231 oxygen sensor upstream TWC # Archived Message

    Posted by Dave Hill on May 27, 2007, 7:14 pm

    I have an X reg Merc C200 Kompressor (WDB203045) that has come in with a MIL on & a code P0231 oxygen sensor upstream TWC.

    I had the same motor in last September with the same code & after checking the obvious I replaced the pre cat O2 sensor (Genuine Bosch unit). This time around I have the benefit of the Pico to look at the detail in the sensor output & here are the results......
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    At first the O2 sensor was seen to be switching, although my attention was drawn to the unusual rapid fluctuation within the normal switch period.
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    After a minute or two of fast idle the O2 sensor voltage can be seen to drift into negative voltage. The CO at this point was 12.5%(idling).
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    I ordered a replacement sensor, which isn’t available until next week, so I fitted a universal 4 wire, just to tide him over until it comes. The emissions are now in spec, although the next capture still shows a tendency for the voltage to go close & actually into the negative again.
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    One other point worth a mention is that the O2 sensor multi plug was soaked in engine oil, which was strange as there was no oil anywhere near it. I suppose oil must be coming through the loom. Is there any truth in that, an O2 sensor receives its reference "atmospheric" gas through the wires? (poorly worded sorry)

    Even the replacement sensor displayed the same fluctuating characteristic when idling. I haven’t seen one like that before & was wondering if anyone has any thoughts on it.

    Cheers all

    Dave (Hill)

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