It may have an exhaust leak messing up the sensor reading.

For LM2, P2C7A is one of the codes listed in this recent article.

Did they set you up with an appointment?

Checking for Exhaust System Leaks – TechLink

gm-techlink.com

Checking for Exhaust System Leaks
September 27, 2022
Several DTCs set in the Engine Control Module (ECM) may be caused by an exhaust leak on some 2020-2022 Silverado 1500, Sierra 1500; 2021-2023 Tahoe, Suburban, Yukon and Escalade models equipped with the 3.0L Duramax diesel engine (RPO LM2). (Fig. 1) The exhaust leak will allow oxygen into the exhaust and result in an incorrect Nitrogen Oxide sensor reading.

Fig. 1


The following DTCs may be set:

• P11D5: Nitrogen Oxides Sensor 2 Exceeded Minimum Learning Limit
• P14D9: NOx Sensor Exceeded Minimum Learning Limit Bank 1 Sensor 3
• P20EE: Nitrogen Oxides Catalytic Converter Efficiency Below Threshold
• P2C7A: Nitrogen Oxides Catalytic Converter 2 Efficiency Below Threshold
• P0421: Catalytic Converter Low Efficiency

There are two different types of leak tests that can be performed on the induction or exhaust system — a smoke leak test where the area being tested is filled with pressurized smoke and the leak is detected by visually inspecting for smoke exiting the system or a pressure leak test where soapy water is applied to the outside of the area being tested and leaks are visually identified by the presence of bubbles forming over the leaking area. When determining which test to perform, consider that with a smoke test, the smoke is often easy to see exiting relatively large leak locations but is harder to see exiting a smaller leak. Smoke also can be removed when it passes through the Diesel Particulate Filter (DPF). Using a pressure leak test, bubbles will form over a small leak, making it easy to spot, but it’s difficult for a soap film to form over a large leak.
To help identify the location of the exhaust leak on the 3.0L diesel exhaust system, perform a leak test using the GE-52250 High Pressure Leak Detector.
Refer to Bulletin #22-NA-168 for details on using the GE-52250 High Pressure Leak Detector to locate the exhaust leak. The cooling ports in the exhaust system (Fig. 2) must be sealed off before installing the inflatable adapter from the GE-52250 machine. (Fig. 3)

Fig. 2


In addition, check the joint between the rear Selective Catalyst Reduction (SCR) and the tail pipe. If there is a large leak at this joint, any smoke or pressure introduced during testing will escape. Seal the joint before testing.

Fig. 3


Using the vapor test and the shop air supply along with a soapy water mixture (Fig. 4), all connections, welds and sensors should be checked for air leaks. These areas include the turbocharger, NOx sensors, DEF injector and clamp connections. (Fig. 5)

Fig. 4


TIP: A leak at the turbocharger vane actuator shaft or the EGR back-pressure valve actuator is considered a normal condition. Any smoke or bubbles from these areas should be ignored.

Fig. 5


Once all leaks have been identified and corrections to the exhaust system have been made, verify that the exhaust is sealed by retesting with the GE-52250 High Pressure Leak Detector.
For additional information, refer to Bulletin #22-NA-168.

 

Have appointment set up for this Friday. What happens if drive for greater than 7 days? This part of the message is what really grabbed my attention. Does this mean that after 7 days of driving the truck will not even run in "limp home" mode"

 

I think it's just trying to get you to not delay too long.

Speed limit warnings are for a specific number of miles

 

Update: Did not drive truck for a week after getting the code. When started up to take into the dealer, the CEL was out. Dealer checked and the code was in the "historical" location but not in when checked. Did diagnostic anyway and it was in specification and as there was no code, there was nothing to reset or check additionally.

 

On way in to work on Interstate, check engine light came on. Gages looked normal and no out of ordinary engine sounds or performance that could tell. Stopped at dealer after work and code P2C7A was set. The message that received is "An issue with the emissions system has been detected. Please service your vehicle within 7 days.". Does this mean that after 7 days the truck will not run? This sounds similar to the consequences about running low or out of DEF. The code is "NOx Catalyst Efficiency Below Threshold Bank 1 Catalyst 2". Has anyone received this code and were replacement parts needed or is this a "software" glitch? With the stated supply problems the truck might be at dealer for some time in case there are not replacements there.

I had this problem. The dealer initially said it was an emissions sensor, but after leaving my truck had concluded it was an exhaust leak and had to replace a bracket. My truck went to the dealer the end of September and is still there due to the part not being available. I am very frustrated with this.

 

where are you located? What dealer? Im in San Antonio no having the same issue. Was the dealer able to provide you with another vehicle to use?

 

My understanding is that Cavender GMC in Weimar has some great diesel techs and for sure know what customer service is.

 

CEL popped on today on the way home. 32* and snowing lightly. Running approx 80mph which is not unusual for the afternoon traffic on this 4-lane. Didn't notice it right away but saw it as I slowed to hit I-75. It may have been on for a while after starting with the temps. I do smell exhaust when remote starting in the morning. No other DIC display. Got home and ran the diagnostics on my iDash and P2C7A came up. Cleared the code and we'll see if "the b***h came back" (sorry, Theory of a Deadman playing on my phone...😀).

 

My '22 duramax got the P2c7a at 2,700 miles. Dealer reflashed the ECU yesterday. The service advisor told me the code comes on because the "sensor readings were out of spec and the reflash will simulate the data to be in-spec"...

I was like..... hmm. So GM is performing emissions fraud?

 

I cleared the code and it hasn't come back. I don't know how tight the software specs are but maybe GM has some wiggle room without violating any emissions standards. After VW got nailed to the wall for their shenanigans, I doubt GM would take that chance. But ya never know.

 

I had the same code 2021 GMC Sierra. Took it in August 2022, they ended up replacing Selective catalytic reduction catalyst (Nox cat converter). No codes since, but not happy with this engine. After the first 6 months my mpg dropped down to 17, right before the check engine light came on it was down in the 15's. I knew for at least 6 months something was wrong with the truck, regen every couple hundred miles, erratic def consumption(still happening except now it isn't really using any), mpg down more and more, took to 2 dealership for other repairs 3 different times total and would ask them to check. Until the check engine light came on, nothing was wrong they said. BS it was messed up for 6-9 months. Still dealing with some kind of slow coolant loss since it was brand new, they can't find a leak. I want to know where the 1.5+ gallons of coolant I've put in has went... Last GM for me.

 

I had a check engine light come on then disappeared after having my 2021 3.0 Duramax Diesel turbo for about a year. It was a split cooler on top of the engine. They have a superseded part they will replace it with. Takes about two months to get. If I didn’t bring it in for them to service and mentioned the code, the coolant would have kept leaking into the engine and then siege it. All fixed now. But it’s a year later and now I have the same emissions code everyone else here had.

 

I have had that code every 3-4k miles .. I just got it today but I didn't take it to the dealer I just cleared it ..

 

A few weeks ago this code came up again. Reset it and also stopped at dealer for more information about it as this has come up again. Said that since cleared it could not read it. Drove away and the next week came in again. Took in and service writer read code P2C7A and locked in. Made appointment for follow up. When took in the result was that the DEF injector was clogged. Had on in stock so replaced it and then ran regen cycle which consumed a bit of fuel. This was replaced under warranty for note. The DEF level was going down so does any know if this injector has multiple small holes? Is this available from Rock Auto? Can this be replaced without going to the dealer and how can the regen cycle be forced?

 

Maybe it's one of the other documented causes for P2C7A this time.

 

Update: The code came back twice and was read by the local dealer both times. Last time made a service appointment. Dropped truck off and picked up a rental for that day and next if needed. Received a text that had replaced the second NOX sensor due to a high resistance reading giving a false reading. Time will see if this if this code comes back or not.

 

Just had same engine code.
Dealer just finished the repair which was a plugged def injector. And there is a service bulletin out. I did notice a significant drop In def fluid just before the check engine light went on. Not sure if related but engine has a slightly different sound now and fuel efficiency seems slightly worse. Have noticed a hiss when accelerating. Dont believe it to be in regen. Dealer said everything appears fine, now that injector is clean and code is reset. I will post later if code comes back. But I can't help but think this won't be the last time this code happens.

 

I received the P2C7A code in my 2021 Silverado after a cold remote start. I reset the code and have not had any issues for the past few weeks.

 

Hi All 2021 1500 RST 28,800 or so miles as of this morning. 2 weeks ago my CEL popped on, pulled the neg cable and let it sit 3-4 hours. Reconnected neg cable. Light went off for 2-3 days or about 75 miles. Light came back on. Something I noticed before the light went back on: the smell of diesel was very strong when idling (remote start cold morning appx 28F). Scheduled an appointment with dealer. This was their first time fixing Code P2C7A. The DEF injector was crystalized. Dealer said there was a service bulletin. I had dropped the car off Monday (12/4) part took 2 days and truck was tested and finished by 5pm on 12/6. The dealer told me they had to test drive it and also make sure there was no damage to the catalyst. I picked it up this morning. I asked the dealer what i should look for in the future and they didnt give me much. Has anyone had any long term issues? Potential damage to the catalyst has me concerned. I do have an extended warranty but with my luck the problem will come up after it expires.

This is my first post and i joined the forum to share my experience.

I live in northern NJ. My garage consists of a 68' camaro, 2024 850i and a gorgeous black 2021 RST Z71 Duramax.

 

I'm having the same issue! I live in North Jersey too and I'm getting both P2C7A code and P1488 code. The check engine light has been going off and on for 3-4 months now. Can't remote start in this cold weather either. First they said it was a gasket/seal that had to be replaced. Took about a moth and a half to get the part due to the auto worker strike. Light went off for a week after that fix and came back on. Then the truck went into shutdown mode because of crystal formation in the DEF system. Got that fixed, of course the check engine light went out during that service call so nothing to check. Took to a private mechanic and of course, light went off 2 days before the appointment but he cleared the history where it was showing up. Now light is on again, and I have an appointment next week. Very frustrating! Love the truck but this issue is pissing me off! Did you get yours fixed finally?

 

I have a 2022 Trail Boss LT with 30k miles and this code hit me today. I also remote started this morning and temp was 28F. I have also noticed a strong diesel smell or better explained almost like a chemical diesel smell since I bought the truck. Really weird. Anyway I have an appointment this Tuesday. So if it is the same problem with the DEF injector I will try to pry more information about the catalyst concern out of my service rep. He is a personal friend of mine so I will get answers if there are any.
I will post a reply when I find out more information.

I live in Virginia.

 

Since last posted, the code came back last week while driving into work. Stopped at dealer on way home to read and confirm it was the same code, yep. Dealer did not reset the code. Set up to drop off this coming Tuesday. Service Advisor stated that since had replaced the injector and the sensor, would contact GM and open a case about it. Light was on from then until yesterday on way home and the light was out. Means that the code had been reset by itself? Just gonna run it until the code comes back and then take it in.

 

Here’s my latest experience with P2C7A. It came on once last summer so I took it to my dealership (like any concerned owner would do). Had smoke test at $700 and 5 days out of the truck and was told there was an exhaust clamp leak and wasn’t covered under warranty. Paid my bill and went on my way. It came on again this past fall. Took it back and dealer did a computer update and covered the service. On Jan 4 it came on again so I did a non scientific test of my own and found the following:

Jan 4-P2C7A came on. I made a note of the code and cleared it with my cheap OBD tool.

Jan 18-Same code came back on. 2 weeks seemed a bit long if this was a serious issue. I called the dealership and again was told to drop off the truck and understand that it would be 4 days in line before they could look at it. I checked with another dealership and was told the same 4 days before they could check it out. I searched the code for LM2 and found the following on NHTSA website

Step 1-Clear the DTCs
Step 2-Perform a NOx sensor #1 reset
Step 3-Perform a NOx catalyst reductant loading reset
Step 4-Perform a DPF Regeneration Procedure and return the vehicle to the customer

I contacted a friend who is a mobile diesel mechanic and asked if he could do this with his OBD tool (since it’s as big as a laptop). He said he could. Jan 18 was a Thursday. I drove the truck to work and back on Friday (84 mile round trip) and parked it without clearing the code at all. The next morning I started the truck so he could do the resets and the light was off. Right or wrong this has caused me to adopt a wait and see approach when it comes to these CELs.

 

I picked up my truck Wednesday night and service said mine was a bad Cat converter. They replaced it and it has been good so far. So hopefully no more issues with this, but we will see.

 

FWIW, I had the same DTC at 27,XXX miles (I had thought it was ~7K miles until I just looked at my invoice) on my 21' 3.0. Dealer diagnosed a bad CAT. The NOX sensor was damaged during disassembly. Both replaced under warranty.

Currently a few miles south of 50K. No problems since the original repair.

 

Must be eh cold weather messing with these trucks. Have had P2381 codes for about the past week or so. Today, on the way to Columbus for a musky show, P2C7A popped up as we got into Hilliard. No DIC warning, just the CEL.. Cleared it with my iDash and it didn't come back on the return trip. Think the truck needed a 3-4 hour, two-way run. Mileage steadily increased all day. Ended up at 30.3 over the last 50 miles. Into a head wind...go figure.

 

My 24 LZ0 went into the dealer with a CEL less than a week after I picked it up. Light came on around 125 miles, handed it off to the dealer at 180 miles. They diagnosed it as a failed NOX sensor. The my GMC App showed now issues and nothing came on the cluster about an emissions system problem. I'm hoping it was a damaged sensor from the factory, not an ideal way to start ownership of a modern diesel. I pray by the time the warranty is up (or even sooner) that a delete will become available.

 

I found the best solution to carry a cheap scanner in the truck and just clear any codes that come up. Then you can remote start and everything too.

There’s one for the LM2 already I’m sure the LZ0 wouldn’t be too far behind

 

Back again... I will be dropping it off for the week to be repaired. I love this truck but I have a feeling I am going to get rid of it if this happens again...

 

CEL lights are a lot more common than you think especially on new vehicles. The cheaper alternative is to have a cheap code reader you can read and delete codes with and monitor yourself before taking it in for servicing. If you trade it for something else you’ll likely just run into a similar issues.
These trucks are also more temperamental in the cold.

 

Of all the CEL lights and codes I've been dealing with, this one popped up this week. As with all the others, cleared the code, CEL went out and "175 miles to 65" warning extinguished. Did not come back when restarting and driving it. smdh...

 

At first the only way i got mine to clear was by pulling the negative cable. Less than 100 miles later it was back. Didnt even bother with that the second time, just went right to the dealer cause all of the same symptoms were there. Heavy ULSD smell idling, especially cold.