Ugh, P0420 on my 2nd set of cats

[Mr T]

Folks:

I replaced both cats at 94,000 miles on my 2008 Denali (6.2). I was able to pass emissions then (2013). No issues till summer of 2015, darn P0420 came on again just about time for registration renewal. However, I went to the dealer and paid to have my ECM flashed with the update that is suppose to make it less sensitive. So I was able to get it passed then.

Fast forward to today (Feb 2017), car has 144,000 miles. Engine light has been on for the past 6 months straight. So my "newer" cats now have 80K on them.

I suppose I have to replace my left side bank cat again. Why would I burn through two cats in 4 years? My car does burn about 1 quart of oil in between oil changes (7 to 8k of miles). I think that's normal for a big 6.2L with 177K, but maybe that oil is messing up the cat.

Thoughts?

 

[Buzzcut]

Where these OEM or replacement catalytic converter's ?

 

[Mr T]

They were NOT OEM (flowmaster) and I wonder if that had something to do with it. However, the OEMs failed at 95K so both OEM and after market have failed. Thus my worry something else in my engine configuration/operation is causing this.

 

[Buzzcut]

That's a tough call, but without any other codes to look elsewhere I'd just change the cat. The only exception I would change the brand. I will say that replacements may not be as robust the manufacturer. Good luck and let us know what you did.

 

[Big Mama]

I have the same engine with 112k and it doesn't burn any oil. Oil getting into your exhaust would definately affect your cats. If you go with new ones ask the shop to give you the old ones so you can check them for oil. One other way might be to pull the O2 sensor on that bank and see if it's got oil on it. Check the upstream first since it's before the cats. Have you checked them before replacing the other cats. I've been fortunate to have the originals still on.

 

[michiganlebnani]

Folks:

I replaced both cats at 94,000 miles on my 2008 Denali (6.2). I was able to pass emissions then (2013). No issues till summer of 2015, darn P0420 came on again just about time for registration renewal. However, I went to the dealer and paid to have my ECM flashed with the update that is suppose to make it less sensitive. So I was able to get it passed then.

Fast forward to today (Feb 2017), car has 144,000 miles. Engine light has been on for the past 6 months straight. So my "newer" cats now have 80K on them.

I suppose I have to replace my left side bank cat again. Why would I burn through two cats in 4 years? My car does burn about 1 quart of oil in between oil changes (7 to 8k of miles). I think that's normal for a big 6.2L with 177K, but maybe that oil is messing up the cat.

Thoughts?

Almost similar but little less severe than yours. On my 2007 yukon 5.3 the 1st CAT (OEM) failed at 65,000 miles when i purchased the vehicle in 2013. We installed a Magnaflow on the driver side and that took care the issue. now i am at 125,000 miles and i received the P0420 code again. so i replaced the downstream O2 sensor and that didn't help, and now its in the shop trying to replace the upstream O2 sensor before i replace the CAT again !

My concern similar to yours, as my car burn 1 quart of oil between oil changes. (6k-7k) i use Mobil1 the unanswered question is if its the CAT again, could that be a reason ?? its weird my previous vehicle was a 2001 yukon had about 170k when i sold it and didn't destroy any of the CATs but it also never burned any oil.

 

[Big Mama]

Sometimes the 0420 code is vague enough to make you think it's the cat or 02 sensor. The code registers unspent fuel before and after the cat. The cheapest fix, if it's the problem is the sensor but that code also covers misfires, which can be bad plugs and so on. Since the problem came back it might be worth while to check the plugs on that bank to make sure all of the fuel is being burned. From there you might find a bad plug wire or any number of things. The question Michigan poses is a good one. Oil getting to your cats will definately affect them. The exhaust flows over and through the honeycomb and relies on the precious metal to neutralize the fumes. If the surfaces are covered with oil they won't work and a failure is coming. Just a side thought. Have you tried any of the fuel treatments that claim to clean your cats too? I've never used them and have no idea if they work as advertised.

 

[NGAneer]

I'd do whatever to try and escape the cat nightmare. My wife's LaCrosse threw this code on a trip and we ended up getting the cat fixed under GMs emissions warranty. Check the GM owners site if you're under 80k, they have to replace it by law.

Nevertheless, swap your downstream O2 sensors, see if the code moves. Change your plugs, see if it reoccurs. After that, you might be out of luck.

 

[Big Mama]

So where are you now? I'm just starting from the front. Checked plug wires, checked plugs, checked O2 sensors, or swapped to see if code changed, side note but worth checking is air filter. Like your lawn mower, not enough air equals fouled plugs. Last thing I'd recommend is what I do when I can't pinpoint a problem. My mechanic will diagnose the problem for 1 hour shop fee which is $60. He figures it out I fix it unless it's a job or tools I don't have. Wish I could be more help.

 

[michiganlebnani]

In my case, I replaced both O2 Sensors on bank 1 erased code drive around for 30 miles or so the code came back. I also changed the spark plugs and the wires since my truck has almost 125k miles so i figured its also worth it to change regardless. here is a photo of what they looked like.

the orange light still on, the next step is to replace the cat on bank 1 so i ordered a universal Magnaflow cat for $136 and its currently in shipping supposed to arrive sometimes on Tuesday, then i will go to one of the local muffler shops to replace it.