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  1. #1

    Default P0420 Code Fixes

    Nice Site Everybody,
    I have a 1999 Suburban 1500 that the SES Light has posted (2) times in the last 3 Days. I ran the Codes with an Actron CP9/35 OBDII Scanner and had
    P0420 (Catalyst System Efficiency Below Threshold (Bank 1) showing up. No other Fault Codes. No performance Issues. No Misfire. No recent Maintenance performed, just lots of Driving. Currently at 158K Original Catalytic Converters and O2 Sensors. I bought it 4 Years ago with 70K on it. From reading through the Postings, I am assuming there are (2) Cat's and (4) Sensors. I dont want to start throwing Parts at it. Maybe someone has had this before and know a good Place to Start Troubleshooting.
    1999 Suburban LT 1500
    158,000 Miles
    :glasses::glasses::glasses:

  2. #2
    Moderator

    Davandy's Avatar
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    Haasdonk Belgium
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    Default

    Catalyst System Efficiency Below Threshold (Bank 1)
    What does that mean?

    <content removed due to claim of copyright infringement.

    By the way Welcome to the site
    Last edited by ChevyFan; 01-29-2008 at 05:22 PM.
    Peter Smet
    Haasdonk, Belgium

  3. #3

    Default Thanks Pete

    I appeciate the quick reply. I have reset the SES 4 times already. It last for about a day then comes back with the same P0420 Code. Me and a freind set up a different Diagnostic Tool and Graphed out the O2 Sensors simutaneously while at Idle and driving. |Both pre-cat sensors are matched but the post sensors indicate Bank 1 Sensor 1 to be erratic. It could be the O2 sensor or it's reading correctly and the Cat is partially plugged. I'm going to swap the Post sensors from left to right and see if the problem follws over to Bank 2. if it dosent follow then it looks like a new Cat for the Left side. Ill let you know the outcome.

  4. #4

    Default

    Any Success / failure swapping the post cats side to side?

    Thanks.
    Jason

    '99 GMC Suburban K1500 SLT 5.7L

  5. #5
    Jr. Apprentice
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    Jan 2011
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    North Carolina
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    Default Well...

    I have almost the exact same truck, with the exact same mileage, and the exact same scenario. I'd love to hear how this one "ended"!

  6. #6

    Default

    Quote Originally Posted by ajwade View Post
    I have almost the exact same truck, with the exact same mileage, and the exact same scenario. I'd love to hear how this one "ended"!
    I have the same problem. I am eagerly awaiting the outcome of this thread!

  7. #7
    Jr. Apprentice
    Join Date
    Jan 2011
    Location
    North Carolina
    Posts
    22

    Default Hey...

    Hey Dantheman96... Keep me posted on what happens with yours and I'll do the same... Have you hooked it to a scan reader? If so, what does it say for your Long Term Fuel Trim (LTFT)... Mine is at 21.88 and I'm starting to come to terms with the fact that it really may be the cats...

    Quote Originally Posted by Dantheman96 View Post
    I have the same problem. I am eagerly awaiting the outcome of this thread!

  8. #8

    Default

    Quote Originally Posted by ajwade View Post
    Hey Dantheman96... Keep me posted on what happens with yours and I'll do the same... Have you hooked it to a scan reader? If so, what does it say for your Long Term Fuel Trim (LTFT)... Mine is at 21.88 and I'm starting to come to terms with the fact that it really may be the cats...
    I haven't hooked mine to a scan reader yet. The other day, while sitting in a drive thru, i noticed a clanging noise coming from beneath my truck. I noticed it because the noise was reflecting off of the wall. I parked and crawled under the truck and found the noise was coming from the rearmost cat that's closest to the 02 sensor. It seems the "guts" of the cat have broken loose and are clanging around. I don't blame them, i would too if i had 200,000 miles on me too! So i know for sure now that it's the cat that's throwing the light. I just ordered 2 Magnaflow aftermarket cats on ebay for $60 bucks a pop with free shipping. This is the best deal i could find, plus they are the heavy duty kind that has more precious metal in them so they will last longer. I'm going to weld them in next week and i'll let you know how it works out.

  9. #9
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    Default

    Please do let me know! I think I'm about to do the same thing... I couldn't find any on ebay. I found some on Amazon though. Spending $150 is do-able... Spending $1500 is not an option...

    I appreciate the help man! Good luck!!

    Quote Originally Posted by Dantheman96 View Post
    I haven't hooked mine to a scan reader yet. The other day, while sitting in a drive thru, i noticed a clanging noise coming from beneath my truck. I noticed it because the noise was reflecting off of the wall. I parked and crawled under the truck and found the noise was coming from the rearmost cat that's closest to the 02 sensor. It seems the "guts" of the cat have broken loose and are clanging around. I don't blame them, i would too if i had 200,000 miles on me too! So i know for sure now that it's the cat that's throwing the light. I just ordered 2 Magnaflow aftermarket cats on ebay for $60 bucks a pop with free shipping. This is the best deal i could find, plus they are the heavy duty kind that has more precious metal in them so they will last longer. I'm going to weld them in next week and i'll let you know how it works out.

  10. #10

    Default

    Ok guys, here's the update:


    I just had my 2 aftermarket cats installed and man what a difference! My mechanic did an excellent job welding them in. Both of my cats were clogged, and it feels like a new engine. Power has quite noticeably increased and so has my gas mileage. The SES light is now off and has been so for the last week. So the problem was definitely the cats. These Magnaflow cats are so worth the money; I would have replaced my stock cats with these even if they weren't bad. My Flowmaster 70 Series muffler sounds alot meaner than it did before. It's even turned a few heads when I stomp it. I'll post pics asap.

    1996 GMC Suburban K 1500 "The Love Boat"
    Magnaflow cats
    Flowmaster catback exhaust
    BFG 285/75/16 on stock 16's
    K&N with Cold Redneck Air Intake
    Monroe Reflex Shocks
    A whole bunch of new stuff -with-
    more to come

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