Emissions issues p0420 and p0430 still even after cat and O2 replacement.

Maine2500HD · May 1, 2022

On 8/3/2020 at 1:41 AM, hysinthius said:

Hey guys,

I'm not sure what to do other than light this thing on fire.

2015 GMC Sierra SLE 5.3 Gas,

Replaced Cats / Y pipe by gm at 79000 miles under warranty that was after they had my truck for a week going through all of GM's guideline tests to get it covered.

At 100K miles Check engine light came back on. Same codes P0420 and P0430.

- Put new plugs and wires in and found that both sparkplugs on the rear cylinders had some of the ceramic missing. Replaced them with NGK 5019

- Replaced plug wires with MSD 32819

- Replaced front and rear 02 sensors with Denso 4940 and 4974

Cleared codes drove it around taking it easy on it and both P0420 and 430 came back within 10 miles

So I bit the bullet and ordered replacements y pipe and cats from magnaflow: Magnaflow 52642 | GMC/Chevrolet | Sierra 1500/Silverado 1500/Suburban/Tahoe/Yukon XL | 4.3L, 5.3L | Direct-Fit OEM Grade Catalytic Converter Federal (Exc.CA)

While I was doing that my battery starting acting up so swapped that out with a AGM.

So being that everything was cleared at the time drove it for about 3 days no issues...guess what codes are back. The odd thing now is that before it would take 5-10 min and the code would come back now it takes about a day or so. But same thing...

P0420 on bank 1 / P0430 on bank 2.

I’ve checked for exhaust leaks and rechecked the bolts on the y pipe and made sure the O2 were tight.

I’m at a loss here...no idea. My local GMC dealership auto parts department are morons ...previous dealings so I don't trust them. Any thoughts? Unfortunately I live in a state that needs the codes cleared (NY)

Any help would be greatly appreciated.

John

When you clear the code (by unhooking battery, if that’s what you were doing) there is somewhere around an hour of “run time” where the ECM won’t set P0420 or P0430. That’s a provision to allow for the cat “break-in” or “run-in” procedure that is supposed to be followed after replacing the cats.

So if you aren’t hitting that hour over three days, it won’t come on. If you are like me and go over that time in a day it comes back. For my P0430, if I use my scanner to clear the DTC it will just come back next time it looks for it (which is during defueling interestingly enough).

Edited May 1, 2022 by Maine2500HD

swathdiver · May 1, 2022

9 hours ago, Maine2500HD said:

When you clear the code (by unhooking battery, if that’s what you were doing) there is somewhere around an hour of “run time” where the ECM won’t set P0420 or P0430. That’s a provision to allow for the cat “break-in” or “run-in” procedure that is supposed to be followed after replacing the cats.

So if you aren’t hitting that hour over three days, it won’t come on. If you are like me and go over that time in a day it comes back. For my P0430, if I use my scanner to clear the DTC it will just come back next time it looks for it (which is during defueling interestingly enough).

Have you followed the shop manual's trouble shooting guide for this? You can get a subscription for it at AllDataDIY and you ought to have a bi-directional scan tool that'll talk to your truck.

timmay56 · June 4

The answer to this is simply K.I.S.S. which means as his son movie people don't know keep it simple stupid. It's very simple solution to this problem change your spark plugs get copper cores because the other ones do not conduct electricity as good as copper but do not use the spark plug gap for your truck think about the old school spark plug gap 0.035 somewhere around there should work well if you make the Gap too big it'll burn your electrodes causing your spark plugs to fail but this will clear up your fuel not being burnt all the way thus eliminating the cat codes. Just remember always when working on vehicles go to your Basics first.