[Tread]

I have a ’16 Camaro 2SS M6 non-NPP, and I had the bolt-on Stainless Works Cat-back exhaust system installed (bolt on) a little over a month ago (so no AFM or NPP valves). The exhaust sounds amazing, but a few days after the install, the check engine light came on throwing a P0420 code: "Catalyst System Efficiency Below Threshold (Bank 1)" (bank 1 driver's side sensor 2 after the primary cat). I took it back to the shop that did the install to check for leaks/all connections, and they didn’t really find anything except that one of the bolts for the connections. they reset the code, but later that day it came back on.

After speaking to Stainless Works, they suggested that something else might be going on such as a bad o2 sensor or bad cat. I took the car to the dealership, and after testing what they said was everything, they found that the bank 1 cat converter was bad, and they replaced it under warranty.

After the repair, everything was fine for a week and a half, but then the same code came back on again. The dealership rechecked the cat to make sure something with the car wasn’t trashing it, and it was fine. They are blaming it on a “backpressure” problem with the exhaust, and they’re refusing to look at the car again unless i put back on the stock exhaust, although with the original repair they told me they checked backpressures and everything looked fine.

I’ve noticed that the light comes only at a particular time while driving. It’s probably about 15 minutes after starting driving. While going >50mph if I start coasting toward a stop without the break or clutch applied as the rpms go down, I’ve noticed they hold briefly at around 1700. The car then gives a lurch/jolt and the note of the exhaust changes briefly before returning to normal.

After being cleared, this has to happen several times before the light goes back on, but it always returns after this kind of situation. This sounds like it might be corresponding to the drive cycles of the car.

I also spoke at length with stainless works about what might be going, and they’re equally confused because none of the many cat-backs they’ve sold has has this kind of problem.

Does anyone have any ideas what might be causing this? Do I need to check again for a leak or a bad o2 sensor? (the dealership said the second time they checked the cat they also checked o2, but i’m not 100% they did).

I know with the increasing complexity of cars’ computers and sensors, even a tiny thing off can cause a problem maybe including a little change in flow. Do you think a tune would address this? If this is a “backpressure” issue, i’m a little confused why it would only appear on the bank 1 side and not bilaterally.