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Another P0420 Question

Discussion in 'Vortec 4.2L I-6' started by A7dollarbill, Sep 19, 2017.

  1. A7dollarbill

    A7dollarbill Member

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    2005 Trailblazer LT
    I have a P0420 code. Graphed both o2 sensors and they both read the same. Thought i had a bad upstream sensor. Changed it and i still have the code. I'll post the graphs of it currently. One is just at idle, the other is approximately 3000 rpm for a few seconds. They are different but I still get the code. I have awful gas milage but i don't seem to have a loss in power. The previous owner drove around for monthes with a bad thermostat before i got it. I'm assuming its a bad cat. Thoughts/opinions.

    The purple line is the upstream.
     

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    Last edited by a moderator: Sep 19, 2017
  2. Chickenhawk

    Chickenhawk Well-Known Member

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    Your sensors look fine, but your cat is cooked. (I am assuming you meant to say that the purple line is your downstream sensor.) The blip in the purple line indicates your cat is failing.

    You also likely have a clogged cat. (Clogged cats rarely throw an emissions code because they can be so badly clogged that you can't do over 40 on the highway, but they can still clean what little exhaust gets through them!) You may feel that performance is not affected, but trust me, when you get a new cat - and you need one - you will be amazed at the increase in power, especially on the highway.
     
    Mooseman and djthumper like this.
  3. A7dollarbill

    A7dollarbill Member

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    Installed a new cat today and a new downstream o2 sensor. Turned the check engine light off. All good for about 20 miles and the light came back on. P0420 again. Anything else that could cause that code. Now I have a brand new upstream and downstream O2 sensor and a new cat. I'll post the O2 graphs currently.

    No leaks anywhere either.
     

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    Last edited by a moderator: Sep 21, 2017
  4. Mooseman

    Mooseman Moderator

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    That is very strange. It's not a problem with the O2 sensors themselves as the code specifically says the cat is not working efficiently so it's sensing something.

    Try putting the old downstream sensor back. Only thing I can think of is that you got the wrong one. Maybe also try disconnecting the battery for 30 minutes to reset the PCM completely.

    It's it happens again, grab a graph of the downstream sensor.
     
  5. A7dollarbill

    A7dollarbill Member

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    Not sure what you mean by the wrong downstream sensor.
    Its a bosch and I believe the upstream one is a walker. I'll try disconnecting the battery. I also didn't notice any difference in power when driving yesterday.
     
  6. Mooseman

    Mooseman Moderator

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    I said downstream, meaning the post-cat sensor. It could be wrong or defective. That sensor tells the PCM if the cat is working right and trigger P0420 if it isnt. Try the old one back there if the reset doesn't work.
     
  7. A7dollarbill

    A7dollarbill Member

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    Tried the reset. Didn't work. 30ish miles after I got the p0420 again.
     
  8. A7dollarbill

    A7dollarbill Member

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    Did some sensor swapping. I hope all the graphs make sense. Any thoughts help.
     

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  9. Mooseman

    Mooseman Moderator

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    The downstream should be steady once warmed up. At idle, it looked like they were identical to the upstream, which would indicate the cat is not working at all.

    This video explains and shows what to look for. I know you said that you put a new cat in but is it possible it's bad? Was it a cheap universal type?

     
  10. A7dollarbill

    A7dollarbill Member

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    It was a brand new direct fit. I suppose anything is possible. The only graphs that were at operating temp were most likely the last two. The current graphs look like the ones i was getting before the new cat. When i bought the blazer, the guy said he had a new cat and sensors on it too. But after that he was (and I was) driving around with a bad thermostat.
     

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  11. Mooseman

    Mooseman Moderator

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    Oh geez, I hope you didn't cook the new cat!
     
  12. A7dollarbill

    A7dollarbill Member

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    He had everything replaced when he owned it. Since i bought it i put a new thermostat and sensor in it. Drove great and no CEL for a couple months. Then i got the P0420 code and have been trying to fix it since.
     
  13. A7dollarbill

    A7dollarbill Member

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    So based on watching the above video i figured out how to get dashcommand to display voltages.
    I logged everything. I'll have to figure out how to export it and load it here. But, completely warmed up, and a constant rpm of 1950, the upstream sensor bounces between 0.080(low) and 0.845(high). The downstream sensor stayed between 0.730 and 0.735.
     
  14. Mooseman

    Mooseman Moderator

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    Sounds right. Don't understand why it's throwing the code :confused:
     
  15. A7dollarbill

    A7dollarbill Member

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    Turned the light off yesterday.
    Currently at 75 miles and the light hasn't came back on yet.
    Fingers crossed.
     

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