THAT DARN P0420 CODE AND WHAT I'VE DONE SO FAR...

[Hetfield03]

I've had a check engine light on for quite a while. I'm not sure if anything that has been done to the car brought this on but I will say that I exchanged the exhaust for an axle back which is in my signature below. After that I had a charcoal canister that needed to be replaced by the fuel tank. That code went away. Over the next year or so the check engine light came back on and I had to replace the O2 sensor Upstream. Then eventually the check engine light came back on and I was told that the O2 sensor needed to be replaced on the downstream side but I could pretty much drive it and it would be all right except the check engine light would stay on.

So I literally drove around for about half a year with the check engine light on and nothing seemed to be problematic. Until recently. The car started driving rough and I was getting a flashing check engine light at a stoplight and I barely made it to my destination. I had the car towed in to a mechanic shop where they read a 420 and 430 code.

They said my coil packs were fine, they wanted to replace my spark plugs which hadn't been done since 2014, and they suggested I do a three-step engine system cleaner with QMI fuel products. The throttle body was cleaned as well as it could be. I myself added a couple bottles of Chevron Techron after the fact with the next couple of gas fill ups.

The 430 code is completely gone. However, the 420 code keeps coming back. I have a little cold reader that hooks up to my phone but I can only get rid of the code for a couple days and then it comes back when the car starts idling rough, or should I say a little weaker?

My question to you all is what could be the chances if this is to do with the O2 downstream sensor or am I looking at replacing the whole catalytic converter? and just out of curiosity, could any of these things have to do with my axle-back or is that a complete non-issue? And could this one could progress to something even worse?

[gtstorey]

How many miles on the car?

[Hetfield03]

Originally Posted by gtstorey

How many miles on the car?

134,000. Thinking its the cat?

[gtstorey]

It could be. Does the car burn oil? Has it been driven with a miss? I would have to read the diagnostic steps for those codes to see what how you can do to determine which it might be. You might be able to compare voltages of the upstream and downstream o2 sensors to see if anything looks wrong with the signals.

Here is a link that list the possible causes of the codes. But we can only guess what the actual cause is. Maybe a bad 02 sensor is the most likely cause, but that might only be 50% of the time.
https://www.700r4transmissionhq.com/p0420-code-gmc/

[xstuntman]

Originally Posted by Hetfield03

134,000. Thinking its the cat?

It wouldn't be a bad idea to do a valve cleaning and then run some cat cleaner thru it.

[Oxtar41]

Originally Posted by Hetfield03

I've had a check engine light on for quite a while. I'm not sure if anything that has been done to the car brought this on but I will say that I exchanged the exhaust for an axle back which is in my signature below. After that I had a charcoal canister that needed to be replaced by the fuel tank. That code went away. Over the next year or so the check engine light came back on and I had to replace the O2 sensor Upstream. Then eventually the check engine light came back on and I was told that the O2 sensor needed to be replaced on the downstream side but I could pretty much drive it and it would be all right except the check engine light would stay on.

So I literally drove around for about half a year with the check engine light on and nothing seemed to be problematic. Until recently. The car started driving rough and I was getting a flashing check engine light at a stoplight and I barely made it to my destination. I had the car towed in to a mechanic shop where they read a 420 and 430 code.

They said my coil packs were fine, they wanted to replace my spark plugs which hadn't been done since 2014, and they suggested I do a three-step engine system cleaner with QMI fuel products. The throttle body was cleaned as well as it could be. I myself added a couple bottles of Chevron Techron after the fact with the next couple of gas fill ups.

The 430 code is completely gone. However, the 420 code keeps coming back. I have a little cold reader that hooks up to my phone but I can only get rid of the code for a couple days and then it comes back when the car starts idling rough, or should I say a little weaker?

My question to you all is what could be the chances if this is to do with the O2 downstream sensor or am I looking at replacing the whole catalytic converter? and just out of curiosity, could any of these things have to do with my axle-back or is that a complete non-issue? And could this one could progress to something even worse?

Chances are the cat has failed. 9/10 times replacing oxygen sensors wont do anything for a p0420 /p0430 code. The cat cleaners and snake oil products don't work either. So you have a few options. 1: replace the cats. 2: get some headers and delete the cats and get a tune. 3. buy a o2 sensor extender to move the post cat 02 sensor up out of the exhaust stream so it doesnt detect that the cat is bad, I have done this option on my daily which is a 07 yukon with 334,000 miles and i wasnt gonna spend the money on replacing the cats on that turd.

https://www.temu.com/subject/n9/goog...ctx=f-28e2324c

[Hetfield03]

Originally Posted by Oxtar41

Chances are the cat has failed. 9/10 times replacing oxygen sensors wont do anything for a p0420 /p0430 code. The cat cleaners and snake oil products don't work either. So you have a few options. 1: replace the cats. 2: get some headers and delete the cats and get a tune. 3. buy a o2 sensor extender to move the post cat 02 sensor up out of the exhaust stream so it doesnt detect that the cat is bad, I have done this option on my daily which is a 07 yukon with 334,000 miles and i wasnt gonna spend the money on replacing the cats on that turd.

https://www.temu.com/subject/n9/goog...ctx=f-28e2324c

My good buddy that usually does the major work on my car suggested doing the extender solution too. Whatever is the easiest thing to do but also wont make the problem worse.

[Oxtar41]

Originally Posted by Hetfield03

My good buddy that usually does the major work on my car suggested doing the extender solution too. Whatever is the easiest thing to do but also wont make the problem worse.

Only issue with that method is if the cat breaks apart it is possible that it will clog the exhaust system. Most of the time the internals break apart and then rattle for a bit and break up into smaller pieces and then blows out the back that's what my Yukon did lol so now I have 1 high flow cat on it lmao.

[Hetfield03]

Originally Posted by Oxtar41

Only issue with that method is if the cat breaks apart it is possible that it will clog the exhaust system. Most of the time the internals break apart and then rattle for a bit and break up into smaller pieces and then blows out the back that's what my Yukon did lol so now I have 1 high flow cat on it lmao.

So youre saying eventually, I would have to do something about the cat anyway? Either delete it or replace it once it breaks apart?

[Oxtar41]

Originally Posted by Hetfield03

So youre saying eventually, I would have to do something about the cat anyway? Either delete it or replace it once it breaks apart?

chances are if / when it brakes apart it will blow out the back and you don't even notice, however i have seen the cats break apart and get lodged in the exhaust and restrict air flow, usually followed by poor drivability because of it. It doesnt always happen that way but I have seen it happen. If I were you id go the extender route, if it clogs the exhaust just drop the exhaust and remove the blockage and bolt the exhaust back up. Easiest way to go about it.

[Hetfield03]

Originally Posted by Oxtar41

chances are if / when it brakes apart it will blow out the back and you don't even notice, however i have seen the cats break apart and get lodged in the exhaust and restrict air flow, usually followed by poor drivability because of it. It doesnt always happen that way but I have seen it happen. If I were you id go the extender route, if it clogs the exhaust just drop the exhaust and remove the blockage and bolt the exhaust back up. Easiest way to go about it.

It probably already did. I know that sometimes I will hear a rumble and Id just think, its my axle back so I never thought much of it. Every so often, like every other day and then it just goes away

But then I read more into it after you posted.

"When a catalytic converter suffers damage, the delicate ceramic honeycomb may become cracked and broken. These loose pieces of converter often rattle audibly when idling or starting up from a stopped position. While this sound may not strike you as anything too terrible, it likely means that your converter has become damaged to the point that it can no longer filter exhaust gases the way it should."

[Oxtar41]

Originally Posted by Hetfield03

It probably already did. I know that sometimes I will hear a rumble and Id just think, its my axle back so I never thought much of it. Every so often, like every other day and then it just goes away

But then I read more into it after you posted.

"When a catalytic converter suffers damage, the delicate ceramic honeycomb may become cracked and broken. These loose pieces of converter often rattle audibly when idling or starting up from a stopped position. While this sound may not strike you as anything too terrible, it likely means that your converter has become damaged to the point that it can no longer filter exhaust gases the way it should."

yep!

[Hetfield03]

Originally Posted by Oxtar41

yep!

Well unfortunately the only code that was completely cleared has come back. Now its showing both 420 and 430. 430 was gone for a couple of weeks.

[Hetfield03]

Cleared the codes this am. Drove 90 miles to Louisville, 90 miles back to Cinci area, check engine light didn't come on...I scanned when I got in the driveway. I got them still in the computer.