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Thread: P0420 Code Question

  1. #1
    Knows a Thing or Two monstrousmac's Avatar
    Join Date
    Jan 2013
    Tucson AZ

    P0420 Code Question

    Ok, so here is the scenario. I originally got a check engine light about a month ago, i turned the key three times and it spit out the P0420 code. So, I knew I had a cracked driver side exhaust manifold for a few weeks. I waited about three weeks to change it. It was in the negative teens in Montana. I changed the exhaust manifold, reset the engine code with my AEV Procal. I drove to Tucson AZ, the code came back. I changed the upstream and downstream 02 sensor. Before I did this the code would come back within a mile of driving.

    After I changed the O2 sensor the code would stay off for a couple of days. Also, when the check engine light comes on and I cycle the key three times the diagnostics just says “done” no code. When I hook up an actual scanner it does say P0420. I could drive on the interstate no code doing 70 mph. When I come off the highway at about 30 mph it came back on. It seems to be longer and longer before the code comes back. I took it to a reputable exhaust shop to replace the catalytic converter, the owner did some checks and found some bolts loose by the manifolds. He said I should not change the catalytic converter, and too reset the code a few times to allow the vehicle to go through a few driving cycles. He said the pressure coming out of the tail pipe is fine and it is not clogged.

    So, does anyone have any ideas? Does it sound right to reset it 4 or 5 times and allow the vehicle to go through a couple of driving cycles. The motor does not run rough and idles fine.

    Forgot, it is a 2010, 3.8 liter with 171,489 miles. I changed the spark plugs a few months ago, with live wire and a screaming demon coil pack.
    Last edited by monstrousmac; 02-12-2019 at 12:53 AM.

  2. #2
    Meme King WJCO's Avatar
    Join Date
    Sep 2015
    If it's not completely plugged, it still could have enough of a restriction for the o2 monitor to trigger a code. Might want to run a couple of higher octane fuel tanks or some BG 44k to see if you can clean any carbon out. No guarantee but worth a try.
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  3. #3
    Fresh Catch
    Join Date
    Jul 2016
    Fletcher NC
    You need to read the data stream. If the PCM sees the downstream oxygen sensor reading flunctuating, it senses the cat is no longer efficient. An efficient cat would make the outgoing exhaust generate a very steady oxygen sensor signal. The upstream sensor will bounce rich/lean, rich/lean but the downstream should be steady.

    If you see the DS sensor signal flunctuating much at all and there are no leaks between the 2 sensors on that bank, you have a bad cat.
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  4. #4
    Knows a Thing or Two monstrousmac's Avatar
    Join Date
    Jan 2013
    Tucson AZ


    Well checked the downstream O2 sensor and it was fluctuating from .155 to .760, the other Downstream O2 sensor was steady at .760. My fuel trim was also high. So, I had the Catalytic converter changed for 300 dollars and every thing seems fine. No, codes or check engine lights for right now.

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