code P2C7A; NOX nox catalyst inefficiency

[BBell]

i have a 2021 gmc sierra 1500 w/ duramax 3.0L.  i have had over 15 check engine lights come on since my truck had about 10,000 miles or less and they all reported a code P2C7A.  in the begining i scheduled service at the dealer since the Onstar reports says to get it in with-in 7 days to correct.  only a couple times was the check engine light still on and at each service they did nothing or reported there was nothing to it.  the check engine light always goes off after a day or 2 and all is supposedly back to normal.  i have noticed that my fuel mileage has dropped over the last few years and now is much lower than it should be.  At my recent oil change this week the check engine light happened to come on just before i went in for service and remained on when being serviced.  i was told that the code was regarding a NOx catalyst inefficiency error.  they did not have an opening to check that out while i was there for the oil change and recommended scheduling another service visit.  This has been a very frustrating issue for the last 4 years for me since it has never been addressed and continues to this day.  I decided to put this info into an AI model and found this is a known issue.  I am very disappointed that the dealership has not know about this & i have been asking about it for 4 YEARS.  how is this possible?  i have pages of OnStar reports with dates and milage showing this error code over the life of my truck.  i call OnStar every time i have a check engine light and save every correspondence.

 

• Service Bulletins: several Technical Service Bulletins (TSBs) and service updates from General Motors that address various issues related to the emissions system, including NOx sensors and catalyst efficiency. These bulletins provide guidance to dealerships on how to diagnose and repair specific problems that can lead to a "check engine" light and related diagnostic trouble codes (DTCs) like P2C7A, P20EE, and P11D5. TSBs are are instructions to technicians on how to fix a known issue.

• Common Issues: The search results show that owners of 2021 GMC Sierra 1500 trucks with the 3.0L Duramax diesel have experienced "check engine" lights and codes related to the NOx and emissions systems. This is a known issue, and it can be caused by various factors, including exhaust leaks, faulty sensors, or even outdated software in the Engine Control Module (ECM).

 

here is the info Onstar sends me each time it occurs:

ECM - Engine System

P2C7A

The Emissions System is not performing as expected. An issue has been detected in the Exhaust Emissions System which monitors and controls exhaust gases released into the air from the engine. If the vehicle is continually driven with this light on, the emission controls might not work as well, the vehicle fuel economy might not be as good, the engine might not run as smoothly and could lead to future repairs.

 

i am quite sure that many other customers have had the same exerience.

how can i get this addressed.

thank you for your help

[dna9656]

Call Chevy Customer Service, 1 800 222 1020.

[newdude]

18 hours ago, BBell said:

i have a 2021 gmc sierra 1500 w/ duramax 3.0L.  i have had over 15 check engine lights come on since my truck had about 10,000 miles or less and they all reported a code P2C7A.  in the begining i scheduled service at the dealer since the Onstar reports says to get it in with-in 7 days to correct.  only a couple times was the check engine light still on and at each service they did nothing or reported there was nothing to it.  the check engine light always goes off after a day or 2 and all is supposedly back to normal.  i have noticed that my fuel mileage has dropped over the last few years and now is much lower than it should be.  At my recent oil change this week the check engine light happened to come on just before i went in for service and remained on when being serviced.  i was told that the code was regarding a NOx catalyst inefficiency error.  they did not have an opening to check that out while i was there for the oil change and recommended scheduling another service visit.  This has been a very frustrating issue for the last 4 years for me since it has never been addressed and continues to this day.  I decided to put this info into an AI model and found this is a known issue.  I am very disappointed that the dealership has not know about this & i have been asking about it for 4 YEARS.  how is this possible?  i have pages of OnStar reports with dates and milage showing this error code over the life of my truck.  i call OnStar every time i have a check engine light and save every correspondence.

 

i am quite sure that many other customers have had the same exerience.

how can i get this addressed.

thank you for your help

 

 

Knowledge about what exactly?  What is the dealer supposed to know about that you think AI is?  AI doesn't have access to GM Service Information.  

 

 

[Grumpy Bear]

15 hours ago, newdude said:

 

 

Knowledge about what exactly?  What is the dealer supposed to know about that you think AI is?  AI doesn't have access to GM Service Information.  

 

 

 

But apparently it has access to the NTHSA TSB list and those documents.

 

Not an AI fanboy myself.

It's like a 70+ year old man. Lots of information and little filtering.  See, I just proved it. 

 

https://static.nhtsa.gov/odi/tsbs/2020/MC-10172812-9999.pdf

 

Edited August 16, 2025 by Grumpy Bear