NEED HELP DTC P0420 on '99 Suburban

southsidesmoka

Original poster
Member
Jul 21, 2018
121
Columbia, SC
To Whom It May Concern:

My truck threw DTC P0420, today. Additionally, it's been carrying P0430 for several weeks. If I'm not mistaken, the P0430 is pointing to the catalytic converter. I ran some Cata-Clean through it with disappointing results.
:eyebrowhuh:
Is the P0420 DTC pointing to a bad upstream HO2S? Has a bad converter caused my upstream sensor to go bad or is this a normal code confirming a bad catalytic converter? Is my situation progressively getting worse? Any and all assistance is greatly appreciated.

Thank you, all, for taking the time to read this. :tiphat:

- southsidesmoka
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
This would mean that both cats are dead. Upstream O2 sensors are not involved in this at all. Downstream are the ones that tell the PCM if the cats are cleaning the exhaust as they should. To confirm, get an IR temp gun and follow along with Eric's video:

 
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Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
Yep. P0420 is for bank 1, P0430 is bank 2, both for cat efficiency below threshold. A temp check would confirm.
 

mrrsm

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Oct 22, 2015
7,639
Tampa Bay Area
A possible Choice for the Y-Pipe with CATS for not TOO Much $$$ (...suggesting also using S/S Band Clamps vs. those "Horse Shoe" U-Bolt Style ...for much better Exhaust Leak Prevention):


99SUBURBANCATS.jpg
 

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