Dreaded P0420 Code :(

[CarolinaGX]

As I evaluate my options, one of which is complete exhaust What are the second set of cats for? They are after all of the O2 sensors so my guess is that the truck doesn't even know that they are there. If I build a new exhaust, starting with DT Headers, do I need 4 cats or can i just do 2 (one on each bank) with the sensors on either side of the cats?

 

[GXer04]

I’d try a Remflex gasket. I had the exact same issue, and an OEM gasket would only last a week or two. A Remflex will seal up the irregularities of an older joint.

I think I used this one:

REMFLEX 8016 2 Inch Gasket Amazon product ASIN B00IQHLJW2

 

[GXO]

As I evaluate my options, one of which is complete exhaust What are the second set of cats for? They are after all of the O2 sensors so my guess is that the truck doesn't even know that they are there. If I build a new exhaust, starting with DT Headers, do I need 4 cats or can i just do 2 (one on each bank) with the sensors on either side of the cats?

I only have 2. All good

 

[CarolinaGX]

I got the P0420 code again recently so I decided to see if it was leaking at the flange between the 2 cats even though I replaced the gasket and a stud. Sure enough it was leaking. So, I took @GXer04's advice and ordered the Remflex gaskets. When I removed the nuts from the flange and poked around to examine the existing gasket, this is what I found:

I did an initial leak test and it seems that the Remflex gasket is working but time will tell.

 

[GXer04]

That looks familiar. My Remflex has been in since 2016, so I think you’ll be alright.