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

hysinthius · August 3, 2020

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

Edited August 3, 2020 by hysinthius
Typos

hysinthius · August 4, 2020

Anyone?

KemCo · August 4, 2020

Look up non-fouler or anti-fouler spark plug cap with code P0420 on YouTube.

You can thank me later.

hysinthius · August 5, 2020

16 hours ago, KemCo said:

Look up non-fouler or anti-fouler spark plug cap with code P0420 on YouTube.

You can thank me later.

yes I am familiar with those. In fact i have some on the bench as a last case. I just was hoping I could fix this properly.

14LTZ-1LZ · August 7, 2020

So you replaced the two sensor before the catalytic convector or the two after?

txab · August 7, 2020

3 minutes ago, 14LTZ-1LZ said:

So you replaced the two sensor before the catalytic convector or the two after?

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

14LTZ-1LZ · August 7, 2020

Missed ready that part. Have you had it hook up to a scanner to see what's the sensor reading are? Here take a look at this:

He mention that when the cats are replaced you have to reset the ECU or convert replaced updated. I don't know if you need to do that with a GM vehicle.

Edited August 7, 2020 by 14LTZ-1LZ

hysinthius · August 8, 2020

Yes I have done all of the above. I need to run to the dump, so I will do that when and when I get home I'll take some pics of the live data and post them. my scan tool is not as fancy as the one above.

14LTZ-1LZ · August 9, 2020

Here another video to look at, hope it helps especially the smoke test.

hysinthius · August 10, 2020

here are some picks of the sensors. I also found a code which for some reason is in a different part of my scanner...im still learning it. its for the MAP sensor. while it doesnt rate a check engine light according to the scanner. I ordered one of those. should be here tomorrow.

Epsilon Plus · August 14, 2020

I would try MAF cleaner and look at your intake tract. Maybe a new air filter or clean out the measured air coming in the front. For some reason the truck is thinking the outgoing oxygen isn't high enough compared to what's coming in or what it *thinks* is coming in. Check clamp and intake manifold seals and make sure the truck isn't sucking in un-metered air.

If all else fails and you want to keep the truck, there's always HP Tuners and it's magic "No Error Reported" wizardry.

Allterrxin · November 11, 2020

Were you able to resolve your issue @hysinthius? I had the same thing on my 2014 Sierra AT. I replaced cats and all o2 sensors, still had a P0430. I finally fixed, it took me about 3 weeks of just trying to figure it out and I learned a lot about cats and sensors in the process lol. One thing to do is to check the graphs on the scanner while you're driving. I bought a WIFI/Bluetooth OBD2 scanner adapter and downloaded an app on my phone that allowed me to record data and graph the data from the O2 sensors and other data. Just because the graph looks good when idling doesn't mean everything is fine. Take a couple of trips and record the data or have someone drive while you look at the graphs.

In my situation, the downstream sensor was dropping more than it should and that's what triggered the code. From my understanding, the code gets triggered because of the number of the fluctuations, not the actual voltage. Downstream O2's are supposed to be more steady with some variation under load. It turns out I had a small miss because the sparkplugs were not good (I had ordered AC Delco spark plugs from eBay at a good price early this year and had someone else put them in, turns out there are counterfeit sparkplugs out there, but that's a story for another time). After replacing the spark plugs and wires with original AC Delco ones, everything has been back to normal. So it turns out I had a small misfire, not enough for the PCM to detect it, but I learned that you can detect these misfires by looking at O2 and Fuel Trim data on your scanner. On my graphs, the downstream O2 sensor voltage would drop low during acceleration, contrary to what I understood, a misfire like that would make the downstream O2 sensor voltage drop because of the additional unburnt oxygen. I thought it would go up because of the unburnt fuel, but not always.

Also, reset the check engine light and when it comes back up, check the freeze frame data. You should be able to see exactly what triggered the trouble code.

Another good indication of this condition for me was poor fuel economy. Check the fuel trim and O2 data, for irregularities. Check to see if your Short Term Fuel Trims spike up past 10% (lean) or so, and see if your downstream O2 drops at the same time. Check that your air filter hose/duct after the MAF is tight and properly attached. If you have more air coming in than what your MAF is telling your PCM, that would cause this issue.

This was an easy way for me to check without throwing money and parts into it and messing something else up. I spent 16 bucks on the scanner and 6.99 on the app. I looked for info left and right and couldn't find anything other than people saying that a P0430 and P420 just means you need to replace a cat, but I found out that is not always the case. Besides, if you replaced your cat, that means something made the cat go bad, so you will still need to fix it or replace the act again sooner than you should.

I hope this helps someone, I spent 3 weeks looking at graphs, and sensors. I am by no means a mechanic, and if anyone else knows better than me, feel free to correct me.

Edited November 11, 2020 by Allterrxin

diyer2 · November 11, 2020

I would put AC Delco plugs and wires on it and replace other parts that aren't AC Delco.

Allterrxin · November 11, 2020

10 minutes ago, diyer2 said:

I would put AC Delco plugs and wires on it and replace other parts that aren't AC Delco.

I would say that too. If you haven't changed them in a while, you could do that. You can also get one or two out from each bank and check the condition and gap. I would still recommend to check the graphs first. Just looking at the pictures that were posted, it looks like the bank 2 is sticking on the high side a bit on the upstream sensor.