[Diamond]

I bought my 2018 ZL1 from a Chevrolet dealership on 23 Jul 21 with the with an AWE cat-back Touring exhaust already installed by the previous owner. The dealership Certified the car (172-point inspection). about a week and a half later the engine check light came on with the P0420 code. I reset it to see if it was just a fluke. 1 week later the light, and code was back. I took the car to another (much closer) dealership and they replaced one of the O2 sensors. I didn't get 5 miles from the dealership and the light, and code was back. I took it to back the dealership and this time they replaced the catalytic converter. I didn't get 2 miles from the dealership and the light was back on. I took it back to the dealership and they "discovered" a clamp on the bank 1 side catalytic converter was missing. When the 2nd (correct) clamp came in i brought the car in this morning to have it installed. They completed the work this morning and charged me $177 for the "missing" clamp. I didn't get 3 miles from the dealership before the light, and code was back.

I contacted AWE about the issue after the catalytic converter was replaced and they tell me they never see any issues or codes with this system. The dealership is now telling me they have done all they can.

My only recourse at this point seems to be for me to find a competent shop that can troubleshoot the issue. I plan on taking it to one next week. Just wondering if anyone on this forum has had this problem.

[est1979]

Same issue, stock exhaust and had just installed a RotoFab Dry filter intake system, RF replaced and CEL is gone.

What intake is on the car, and have you checked your MassAir connection?

[KingLT1]

That code normally means there is a leak in the exhaust up by the second o2 sensor on bank 1. my guess the leak is at the slip connection.

[Diamond]

Originally Posted by est1979

Same issue, stock exhaust and had just installed a RotoFab Dry filter intake system, RF replaced and CEL is gone.

What intake is on the car, and have you checked your MassAir connection?

Thank you for your speedy response.

I installed a normal Rotofab (dry filter) after the catalytic converter was ordered by the dealership. The issue, and symptoms are the same before and after.

[Diamond]

Originally Posted by KingLT1

That code normally means there is a leak in the exhaust up by the second o2 sensor on bank 1. my guess the leak is at the slip connection.

Thank you for the speedy response.

I will have the shop look at the connection. Can you tell me if this is a connection done at the factory or could this connection have been altered when the AWE system was installed?

[Blueclyde]

Originally Posted by Diamond

Thank you for the speedy response.

I will have the shop look at the connection. Can you tell me if this is a connection done at the factory or could this connection have been altered when the AWE system was installed?

It very likely is the connection where the AWE system attaches to the primary catalysts. There is a post catalyst EGO sensor close to that connection. A slight leak can draw oxygen in (pulsations) and have it influence the post O2 sensor reading during an OBD monitor diagnostic. I too would be looking for an exhaust leak that the connection point of the AWE system is where I would start. It doesn’t need to be a very big leak either.

[BlaqWhole]

Just to confirm, you do have a total of 4 catalytic converters, right?

I ask this because with a catback you have to actually cut your stock system. It is possible that the rear cats were removed by the previous owner. And it is further possible that the dealership did not realize that there should be 4 instead of 2 IF the rears were removed.

Therefore my suggestion is to crawl under there and make sure you have 4 cats (2 on each side). If you don't have 4 then do not inform the dealership of that as that will void your warranty. At that point your best bet would be to get sparkplug defoulers to use as spacers on the O2 sensors as that might be enough to trick the O2 sensors into thinking that everything is fine. Other than that I am not sure what else could be the issue.

But again, it could be possible that the rear cats were removed at some point and it slipped right under the dealership's nose. If they don't service these cars often or are not completely familiar with them or were perhaps super busy and overlooked it (intentionally or unintentionally) then that would explain it. And it would explain why they can't resolve your issue.

[KingLT1]

Originally Posted by Diamond

Thank you for the speedy response.

I will have the shop look at the connection. Can you tell me if this is a connection done at the factory or could this connection have been altered when the AWE system was installed?

It's likely where the AWE system slips on to the primary cat pipe. Driver side is a slip connection(dumb ass design) and passenger side is a flange connection with gasket(how both sides should of been).

Likely what will need to happen is the clamp will need to be removed and clean surface. Apply a bead of Permatex Ultra copper around the slip connection, then clamp with a 2 3/4" 2 bolt lap joint band clamp. Or just have it welded....

Originally Posted by BlaqWhole

Just to confirm, you do have a total of 4 catalytic converters, right?

I ask this because with a catback you have to actually cut your stock system. It is possible that the rear cats were removed by the previous owner. And it is further possible that the dealership did not realize that there should be 4 instead of 2 IF the rears were removed.

Therefore my suggestion is to crawl under there and make sure you have 4 cats (2 on each side). If you don't have 4 then do not inform the dealership of that as that will void your warranty. At that point your best bet would be to get sparkplug defoulers to use as spacers on the O2 sensors as that might be enough to trick the O2 sensors into thinking that everything is fine. Other than that I am not sure what else could be the issue.

But again, it could be possible that the rear cats were removed at some point and it slipped right under the dealership's nose. If they don't service these cars often or are not completely familiar with them or were perhaps super busy and overlooked it (intentionally or unintentionally) then that would explain it. And it would explain why they can't resolve your issue.

Usually these codes don't pop up unless the rear cats are removed. Reason being if the exhaust is cut behind the secondary cats it's far enough back the rear o2 sensors don't detect it even if there is a leak.

[BlaqWhole]

If all 4 cats are present then the only other thing could be that perhaps you have a defective O2 sensor. Even tho it is new, an O2 sensor can read faulty. I've seen some cases where a person went thru several new O2 sensors and they all were defective. Dorman...

It could also be a problem with the ECU.

Or it could be that if the cats are there then they might be gutted or have a hole or crack somewhere.

[BlaqWhole]

Originally Posted by KingLT1

Usually these codes don't pop up unless the rear cats are removed. Reason being if the exhaust is cut behind the secondary cats it's far enough back the rear o2 sensors don't detect it even if there is a leak.

That is my thought. That the rear cats were removed and the dealership didn't know any better. Perhaps OP is thinking rear cats and the car actually only has the 2 fronts...so when the dealership said they replaced it they were talking about the front and OP didn't realize it. Like there was a miscommunication.

That is the only thing I can think of that would make sense.

[Blueclyde]

Only the front catalysts are monitored. The rear catalysts can be removed, and if done properly (no leaks), the system will not set catalyst efficiency codes.

[Stephen12ZL1]

it is a leak at the slip connection on the driver's side as described above...I had this happen on my '17 when there was a small leak there. Even a "mouse fart" leak will cause this code to appear...easy fix.

[Spaceme1117]

I would recommend replacing your Rotofab clamps with some Mishimoto constant tension clamps. I had problems with the couplers sealing with my Rotofab even though the clamps were tight. I installed the Mishimoto clamps, disconnected the battery for a half hour to reset the computer. No problems since.

[Diamond]

I really appreciate all the great comments. I have been a member for some time now and this is the first time I didn't really see an issue I was experiencing. I will take the car to an exhaust shop and have them reseat and re-clamp the AWE pipes to the converters.