P0420 on 2000 Montana

Discussion in 'Domestics' started by JP, Jan 18, 2013.

  1. JP

    JP Hero Member

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    Same one, 90K miles.

    P0420 Cat code comes up every few days; I read it and erase it, then a few days later it lights up, seems like always at idle.

    Needs a new cat, no? Or will soon? I'd just ignore it, or fix it with a little piece of electrical tape over the MIL ;D but if any other code sets I wouldn't know about it.

    My life has been a constant parade of MILs (not MILFs :eek:) lately.
     
  2. Mobile Dan

    Mobile Dan wrench

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    A lazy old upstream O2 sensor may sometimes cause that code.
     
  3. Gus

    Gus wrench

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    The only way to know for sure is with a scan tool...

    You observe the waveform of the precat O2 sensor and compare it to the waveform of the postcat O2 sensor.....

    The pre should be constantly changing.....the post should change slow, staying high for awhile and then staying low for awhile.......that's normal....

    When the rear O2 sensor "mirrors" the front sensor(meaning it looks the same), then the cat is no longer using oxygen and is not working.....you need a cat........
     
  4. JP

    JP Hero Member

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    How could I tell if it was an upstream O2 sensor going bad i.e. "lazy"?
     
  5. Mobile Dan

    Mobile Dan wrench

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    Typically o2 sensor total range is between 0.000 volts and 1.000 volt (1 volt). With sensor at normal temp, reading should cycle between 0.200v and 0.800v. A lazy sensor may only cycle between 0.400v and 0.6.00v.
     
  6. JP

    JP Hero Member

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    How would that show up on the waveform? I have capability through Enginuity software to real-time the waveforms....not sure if it reads out the voltage over time or just what.

    In any case, I reset the MIL and it's been off since then. One thing is for sure, it will be fine right up until it decides to act up again. I'll check it when it warms up a bit outside; it was -4 overnight and 18 today here. Coldest we've had in a while.
     
  7. Mobile Dan

    Mobile Dan wrench

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    If you mean Autoenginuity, you can read o2 information at least 6 different ways. My keyboard doesn't do graphs. :-[
     
  8. JP

    JP Hero Member

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    Yes, Autoenginuity. I'm thinking of the real time graphing. So what am I looking for in a lazy upstream O2 sensor?
     
  9. Mobile Dan

    Mobile Dan wrench

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    Here are some graphs on page 5.

    http://www.autoshop101.com/forms/h37.pdf
     
  10. JP

    JP Hero Member

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    Looks like what I'm looking for is a tall waveform rather than a flattened one or in other words a wide variation between the peak and the nadir of the readings, and the downstream sensor should mirror the upstream sensor except by being delayed?
     
  11. Mobile Dan

    Mobile Dan wrench

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    I'm not sure "delayed" would be the best term. How about "muffled"? The peaks should rise less and the valleys should dip less. When they look pretty much the same (mirrored), that's when the P0420 code is flagged.
     
  12. billr

    billr wrench Staff Member

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    Won't the after-cat sensor switch less often, another difference in wave-form?
     
  13. Gus

    Gus wrench

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    I don't think a "lazy" O2 sensor1 can cause a P0420......if there is a problem with the O2 sensor1, it should set it's own code, and if present with the P0420, that code should be taken care of first.....

    Yes, Billr, that second wave form will change much slower......the first one is constantly changing due to A/F ratio being changed due to the PCM feedback.....the rear O2 does not if the cat is working correctly, because sometimes the cat will be using the oxygen, and sometimes the cat will be storing the oxygen.......this process is much slower, causing the waveform to be much slower.....
     
  14. JP

    JP Hero Member

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    OK I did some monitoring, but still have a question on interpreting these readings. Not sure if I need to look at driving vs. idling.

    At idle, The pre-cat O2 seems to vary "normally", but the post-cat is almost flat line with an occasional blip here and there.

    While driving, the pre-cat readings vary between 82mv at a low and the high 700mv and low 800mv. Post-cat is from the 60s mv to low 200mv with an occasional blip into the 300-500mv range.

    When driving, the two live data graphs look similar at times, but are farther apart at others. The post-cat never gets up in the the pre-cat readings.


    Not sure how to interpret. I do have an exel printout of the readings if that is helpful.
     
  15. billr

    billr wrench Staff Member

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    Are the two O2 sensors the same P/N? I would swap them, see if results change (follow the sensor). It seems it is either a bad after-cat sensor or bad cat. Another test would be to measure temperature on the exhaust pipe on both ends of the cat. A properly operating cat will be generating heat as it converts the unburned HC, exit temp should be at least 100F higher than inlet.
     

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