[Ironsights]

So I just installed the Stainless Works catback on my ‘16 SS and within a few weeks, the CEL came on. Code P0420. I’ve checked and there are no leaks, everything is torqued properly. I cleared the code and within a week the CEL came back on. I’m well aware that this system removed my secondary cats but I haven’t heard of anyone having CEL’s after removing the secondary cats. I was under the impression the secondary cats were not monitored. Any thoughts on this? I’m guessing I’ll need to weld the cats back on since the code is stored which will raise flags come smog time (California resident).

[enzia35]

Shoot, I still have my secondary cats and still got the code.

[seethruya]

Originally Posted by Ironsights

So I just installed the Stainless Works catback on my ‘16 SS and within a few weeks, the CEL came on. Code P0420. I’ve checked and there are no leaks, everything is torqued properly. I cleared the code and within a week the CEL came back on. I’m well aware that this system removed my secondary cats but I haven’t heard of anyone having CEL’s after removing the secondary cats. I was under the impression the secondary cats were not monitored. Any thoughts on this? I’m guessing I’ll need to weld the cats back on since the code is stored which will raise flags come smog time (California resident).

When I removed the secondary cats, I kept the stock piping. Had everything welded at a shop. Couple days later, I got the CEL. About a week later, you could hear a faint exhaust leak. A month later, went to a different shop and replaced all the piping from the main cats back with 3” piping from a kit and never had an issue again with the CEL. More than likely, you still have an exhaust leak somewhere in your exhaust system.

[Joshinator99]

Originally Posted by seethruya

When I removed the secondary cats, I kept the stock piping. Had everything welded at a shop. Couple days later, I got the CEL. About a week later, you could hear a faint exhaust leak. A month later, went to a different shop and replaced all the piping from the main cats back with 3” piping from a kit and never had an issue again with the CEL. More than likely, you still have an exhaust leak somewhere in your exhaust system.

My thoughts exactly. Every time this topic comes up, sooner or later the OP finds a tiny leak in the exhaust system.

[Spaceme1117]

As far as I can tell, the rear O2 sensors would only be monitoring the front cats not the secondaries.

So the only way you would be getting a P0420 is if there is a leak at the connection to the front cats.

Even if things are properly torqued, it can still not seal perfectly.

I would get some Permatex 81878 Ultra Copper Maximum Temperature RTV Silicone Gasket Maker and apply it to the connections.

[Joshinator99]

Originally Posted by Spaceme1117

As far as I can tell, the rear O2 sensors would only be monitoring the front cats not the secondaries.

So the only way you would be getting a P0420 is if there is a leak at the connection to the front cats.

Even if things are properly torqued, it can still not seal perfectly.

I would get some Permatex 81878 Ultra Copper Maximum Temperature RTV Silicone Gasket Maker and apply it to the connections.

Correct, secondary cats are not monitored.

[Ironsights]

Originally Posted by Spaceme1117

As far as I can tell, the rear O2 sensors would only be monitoring the front cats not the secondaries.

So the only way you would be getting a P0420 is if there is a leak at the connection to the front cats.

Even if things are properly torqued, it can still not seal perfectly.

I would get some Permatex 81878 Ultra Copper Maximum Temperature RTV Silicone Gasket Maker and apply it to the connections.

Prefect thank you. There was no gasket or sealant applied on the passenger side flange so I’ll start there.

[Chris C 1SS 1LE]

Went through this myself some several months ago. When I put the 60606 on, followed instructions (right...) that had me remove the braded copper like gasket on driver's side. I'd presume your code did ID the left bank? So, learned that is what causes this code/left bank. Got a new primary cat assembly installed (the code prompts for replacement of the primary cat....) and of course it comes with that braided coper like gasket. About 3,500 miles or so, no issues and just passed smog / OBDII read here in GA the other day as a requirement for plates. For which they taxed me AGAIN for plates, three vehicles, I'll not get off topic...

And FWIW and best I can tell or recall the code is virtually 100% bank 1, driver's side.

[lt4camaro]

Exhaust leak on the drivers side cat connection is the most usual spot. The right side is a flange with a metal shim gasket, that side should be OK if a new shim gasket was used. On the left side I have had luck with a exhaust wrap type sealer around the pipe and then slide the 2.5 inch clamp on and tighten