[Hike208]

Hey everybody!

I'm getting started on a head scratcher, but thought I could ask for help before getting over the tips of my skis. (Wait, I'm already wayyyyy over them.) Apologies in advance for the long post!

We have a 2016 Colorado CCLB with about 66,000 miles. I am getting the Exhaust Fluid Quality Poor warning and countdown. It came on about 20 miles from home yesterday while we were driving at highway speed with an ambient outside temperature between 0 and 10 degrees Fahrenheit. We stopped and picked up a bottle of fresh DEF and put it in when we got home, just in case. The position 2 NOx sensor was replaced under the now-expired powertrain warranty in September 2021, about 10k miles ago.

We have seen an intermittent P20EE code before, but the CEL is not currently lit, nor is that code currently pending, just the DEF light and the depower message in the DIC.

I got @Snipesy 's Gretio app last night so that I could pull codes specific to the exhaust system. All I have is a cheap ELM OBII Bluetooth dongle that I've used with DashCommand, but an OBDLinkMX+ is on the way and should be here Thursday. My cheap dongle will pull information from what I can tell for a few minutes, but then it seems that it gets overloaded and the data connection drops out.

Following this troubleshooting document from Snipsey I think I have started a Reductant System Malfunction Warning Service Bay Test. I start the test in the Gretio, I see data stream into the data grid for a few minutes, then my crappy dongle, well, craps out. I don't get any other feedback from the app or the truck, so I'm assuming that the test is running. But here are a couple of the interesting things that I saw. Granted there are probably other 'interesting' readings in there, I'm just not smart enough to see/understand them.

• The Nox Sensor 2 concentration is always a few ppm OVER Nox Sensor 1. For example, Nox 1: 82.9, Nox 2: 88.1. My understanding was that Nox 2 should be 85%ish less than Nox 1 when not doing a regen, so this is a first red flag.
• There are many 'Malfunction' values for the various 'reductant heater' parameters. FWIW I live in a cold climate, and was taking the readings at about 30 degrees Fahrenheit.
• The 'Injector Inhibit Reason' returns 'Nox Catalyst Temperature'
• There are a large number of sporadic 'malfuction' values that get returned for ALL parameters related to the Nox sensor 2 (Signal Circuit Open Test, Signal Circuit Shorted Test, Heater Circuit Shorted Test, etc.)
• DPF soot accumulation is stated at 30%
• Nox catalyst efficiency is stated at 80% (I don't know if that's good or not good)
• When I turn the truck off, the Nox 1 reading immediately went to 0, but Nox 2 stayed at 98.8. Not sure is this means anything, or is just an artifact of my crappy reader.

I started the Reductant System Malfunction test then not touched anything for 30 minutes, and the DEF light and DIC message persisted. The CEL does not illuminate and there are no pending codes that I can see.

I have not yet initiated a service regen since I live in a condo complex and it sounds like forced stationary regens are LOUD. I'll wait to do this during the workday when folks are gone. I understand the DPF and SCR systems are unrelated, but it sounds like a regen could burn off extra DEF that may have been injected during an 'Ammonia Slip' feedback loop event?

We are about 80 miles from our closest dealer that we've had poor experiences with, and 150 miles from one that has been decent to us, so I am afraid of eating into my limp mode mileage allowance to just drive around to see if it fixes itself. But I will be making an appointment with the further one as soon as they open this week.

I have read somewhere on the forum that if you pull the connector for Nox 2, the ECM will interpret this as a 'hard' sensor failure and won't start the countdown. Does anyone know if this is true? Could I use this method to get to a dealer?

With all of that information above, what should my next steps be? Some of the red flags seem to point handily to the Nox 2 sensor, but what is the likelihood of it dying less than 6 months after being previously replaced?

Thank you for taking the time to read this! I'm new to diesel and am trying to learn and do as much research as I can on my own, but I know I could use a hand from the smarter folks out there

 

[Hike208]

That looks awfully like a NOx 2 failure. Which sucks because those are in short supply right now.

That's what I'm speculating as well. Don't I know it! Our truck was at the dealer for 3 weeks and it was a knock-down-drag-out fight when NOx 2 was replaced a few months ago. Not fun.

The cheaper elms fail randomly with the app... Just like as you describe.... Which is really unfortunate. The adapter tends to just reset itself and that really messes with things.

And this ELM is about the cheapest one you could get. I think it was $11 in 2015. I have an OBDEleven in our VW car, so it's time to step up to something better for the truck (OBDLinkMX+). Too bad Prime delivery to rural Idaho is 4 days, not 2

And thanks for building the app! I've got a lot to learn about both the program and about the truck. This is a great cost effective way of doing just that.

 

[Hike208]

Sounds like you have bad DEF fluid. You have to drive your vehicle I would say for about 15 to 20 mi to get the fresh fluid to get up the injector and the mileage of good readings to reset the derate message.

Good to know, I've been wondering how much actual driving the system needs to see to go 'Alright we're cool, not gonna offend the EPA today, I'll let you proceed to your destination.'

How much fluid did you have in it before adding the new fluid.

I do not know for sure, I only got Gretio (and the ability to see that info) after I put in the new DEF. But it did drink the entire box of new fluid. Now the reading is 'Full'.

The previous fluid has/had been in there since about Thanksgiving and appears to have been fine, but it has been in the negative teens here at night for the last month or so. Unfortunately the truck lives outside at night. Do we know if prolonged cold exposure like that could cause issues? From some other posts out there it sounds like a 'maybe'.

 

[Hike208]

but, you would get a check engine light if one of them was bad. The SCR and NOx sensors are going to take longer to heat up in the cold.

Gotchya, the CEL didn't pop, so hopefully the heaters are good? Although I was getting a number of 'Malfunction' readings on some of the 'Reductant Heater' parameters.

You should always check the date code on the DEF bottle some stores store keep it too long on the shelf and it should not be in direct sunlight.

I may not be a smart man, but I always check the date codes on the boxes in the store 😄 Being in a more rural area we have a fair number of diesels running around, and I believe the shop I buy DEF from goes through pallets of the stuff.

I should also talk about this. The SCR has to be warmed up obviously, so you'll see this a lot. There is also a "DEF Frozen" inhibit reason.

Copy that, thanks. When I initiated the malfunction test (at least I think it started), DEF temp was above 19F, and oil and coolant temps were in the normal operating zone. But it sounds like maybe I shot the gap and the SCR system and its components weren't as hot as they needed to be to really get to work.

 

[Hike208]

Or it was warmed up. Sometimes these PIDs show the last reason... Meaning they are a historical reference.

If the vehicle wasn't ready for the test it would pop up an error.

Ah, okay, that makes sense it would just store the last entry. I did my best to follow the documentation, but I'm not hanging my hat on anything working as it should on account of my cheapo reader. (This is 100% my problem, not an issue with the app.) Very much looking forward to the OBDLink getting here...

Probably a dumb question, but is the NOx Sensor Reset command in the ECM control section for relearning/setting reference voltage when a new sensor is reinstalled? Or is it some kind of other reset command that could be worth trying in my case? (Sorry if this is a dumb question, again I'm over my skis on all this.)

 

[Hike208]

So why does a reset exist at all? Again, I don't know. Nor does GM's manual really say what it does.

Roger that. Granted, I know that I know nothing, but the more I learn about GM's processes and designs, the more I'm like:

 

[Hike208]

Do you remember what brand/type of fluid that was?

Yep, it was a 2.5 gallon box of Peak Blue DEF (the regular stuff, NOT the Platinum juice) from O'reilly's. It's one of the very few places you can get DEF in my area, and they sell a ton of it. My friend who also has a Colorado Diesel gets his there and has never had an issue.

 

[Hike208]

Thanks. I didn't want to mention the Platinum stuff to not affect your answer. I've heard of reports of that causing issues, but am skeptical. But you're clearly not that.

I'm also on the skeptical side. But if buying the regular stuff means that I get to save a few bucks while not taking a chance, that's what I'm generally going to go with.


Anybody know for sure if pulling the connector for the NOx 2 sensor will disengage limp mode? I understand I'll probably get a CEL for a U029E code or similar denoting a sensor comms issue. Obviously the best way to figure it out would be to simply unplug the sensor, drive it and see, but I'm still afraid of eating into my limp mileage allotment if I need to drive it 150 miles away for repair :/

Thanks!

 

[Hike208]

I am pretty sure a DTC present for the sensor that caused the derate message will not get around the derate anti tampering sequence. That antitampering derate programing is setup by EPA specs for how each stage of it works, not GM.

I figured that it wouldn't be that easy Because, why would it.

Like many others here, I'm all about reducing emissions, but I need our truck to be dependable, reliable, and not be worried about getting stranded in one of the huge western states that we regularly traverse. If I need to modify our exhaust system or programming to gain that peace of mind, I guess that's something I'll need to start looking into.

 

[Hike208]

@Hike208 - I was the one that originally suggested that pulling the NOx2 connector might clear the countdown. My NOx2 went hard fail while on the road and the countdown reset. I did not actually try pulling the sensor.

I thought so! I was having trouble finding that post to link out to. Understood, thanks for the clarification.

I have learned since that NOx2 houses the end of line termination for that particular comm line stub. I don't remember what is on that line besides both NOx sensors but standing waves will be bouncing around on that line without NOx2 and that could trigger additional communication errors.

Makes sense, I think. I'm speculating on things I don't understand here, but I wonder if that comm termination happens at the little processing box that's mounted on the frame rail, or at the sensor itself? In which case I wonder if pulling the sensor side connector (if it can be done, I haven't looked too closely at that side do see if it can be non-destructively removed) would trigger a hard sensor fail, but without comms issues for other sensors on that line?

Or on the extreme end, what would happen if you removed the sensor from the exhaust system and just let it hang and read ambient air while driving? I presume it would bring back a reading wayyyyyyy lower than NOx 1, but maybe it would be so low that the ECM would determine it as an out-of-bounds value and toss a code? Just thinking aloud at this point.

If you do decide to try this - certainly let us know the result.

I'll be sure to report back on whatever happens!

 

[Hike208]

The termination to the box looks a little connector-ish but it is actually a hard wired strain relief.

Thanks for the clarification! Truck is currently parked in a slush puddle, otherwise I would have slid under for a look.

 

[Hike208]

The OBDLink MX+ showed up in the mail today, and it's worked a treat so far with Gretio.

I was able to kick off the Reductant System Malfunction Warning Test, and the engine ramped and pegged 2,000 RPM on the nose for 30ish minutes.

During the test, I did notice that NOx 2 was reading lower than NOx 1, so at least that is moving in the right direction. When the DEF injector was flagged ON, NOx 2 was reading +/-50% lower than NOx 1. I know this isn't the 85%ish reduction we're probably looking for. NOx 2 still has a bunch of Malfunction flags on its other parameters, like the heater and circuit tests.

There were also a number of Reductant Heater and Reductant Injector PIDs that would occasionally swap between OK and Malfunction.

After the test completed the DEF light and the message in the DIC remain on. There is no CEL, nor can I pull any DTCs. There are a ton of 'Unfinished Tests' in the ECM section in Gretio, but I'm not sure if those have any bearing on this situation.

TL;DR, I'm still just as confused as before, and still have the DEF quality message.

Screenshots are of various PIDs throughout the test while it was running.

 

[Hike208]

Did you check “Reductant System Malfunction Warning Service Bay Test Abort Reason”

If it says “NOx Sensor Operating Conditions Not Met” or something along those lines then it’s probably due to the NOx sensor heater.

If it says “None” then it could still be the NOx sensor….

Also the truck should save that from last run so you shouldn’t need to run it again.

also I just realized I'm an idiot and forgot to add that PID to the service bay test list.
It's there now.

Son of a biscuit, I didn't check that PID while the truck was still running after the test. I just went and checked and got a value of 'None', assuming that the value was still stored after a power cycle.

Interesting thing though is that when I just fired the truck up to pull the above PID, NOx 2 was reading 0.00ppm. I also noticed that the Heater Mode for NOx 2 was 'Off' and NOx 2 Heater Operation was 'Not Allowed', while everything for NOx 1 was looking good.

This is starting to smell more and more like the dead heater in the recently replaced NOx 2, although I'm still a bit concerned by some of those Reductant and Injector PIDs occasionally coming back as Malfunction.

 

[Hike208]

Honestly most trucks have failing heaters. A quick search of this forum shows that. I wouldn’t worry about that one.

I’d suspect either the NOx sensor 2 or a plugged injector. The NOx readings you gave were a bit on the high side for #2.

When running the test you can wait for the injector to run. At that point NOx 1 should be around 140 to 200ish.
NOx 2 will then drop, many times to like 5ppm. But throughout the test you may see it bob up and down even into the 50s.

It’s also best to not forget about our lord and savior: The service Regen. While it will not clear the issue the higher overall exhaust temps can revive sensors, burn soot on the injector, etc… While the front portion of the exhaust doesn’t get to Regen temps, it’s still pretty hot.

This is all great info, thanks for sharing!

I wouldn't worry right now about the DEF heaters for now until you get the Nox problems figured out. Strange that there is no DTCs for Nox 2, it has errors for both the sensor and sensor heater. Did you check pending DTCs? Also the control module connected to the DEF system heater and pump is under a cover on the bottom of the DEF tank where the pump access is. The one you see on the frame rail is for the trailer connection and lighting. Nox 2 sensor is only supplied voltage for the sensor and heater along with canbus communications to the ECM.

That's where my attention is as well right now. That's the weird thing, is there are no pending codes in the system either. Not even the P20EE that I've seen come and go. This is great knowledge, thanks for taking the time to share!

 

[Hike208]

So here's the latest: We had an appointment at the nearest dealer we've had a good experience with, which is 150 miles away. (I'm still looking for a diesel independent in my region who is familiar with these systems.) We took two cars, as we anticipated having to leave the truck at the dealer. My wife was in the truck and I was in our GTI that she hates driving because it's a manual.

About 20 miles of highway driving into the trip she called and said that the DIC had cleared, there was no CEL, and everything looked good. Those 20 miles were at an ambient temp of about 15F. We pulled over and I opened up Gretio and saw that the truck had run a regen during those previous 15 miles. Unsure if that 'revived' NOx 2 as @Snipesy suggested it might, but it's an interesting data point either way. We decided to go through with the original plan and drop the truck off at the dealer (we needed to do some errands in the 'big city' anyways).

The next day the dealer calls and says that the tech had been working on it for the better part of 3 hours, including a 30+ mile drive, and they weren't able to pull any codes or other meaningful data to indicate a failing component. The service manager was very nice, but said that there's not much more they can do. The dealer didn't charge us a penny for the inconclusive diagnostic, so they scored some points with me there.

Drove the truck around town and back home for about another 200 miles total the next day without any hiccups.

Although they didn't find anything conclusive, I'm still betting that there's a gremlin in there somewhere that's teetering on the edge of total failure. I'll be keeping an eye on things whenever I drive the truck, and I might just order a NOx2 to have on hand for when the one in there fails outright.

Thanks to everyone for the suggestions and help! As someone who is starting at zero and trying to learn how these systems work, the community here has been incredible.

 

[Hike208]

The plot thickens.

Now we have a DTC out of the ECM, but no depower message in the DIC (thank gawsh). The code is P20EE, and I did not clear it. This is the same code that has been thrown both before and after the previous NOx2 sensor replacement about 6 months ago, and sporadically since.

The truck continues to drive fine. On startup, I get readings from NOx1 almost immediately, but NOx2 stays at 0.00ppm for a little while. I presume this is because the sensor heater should be doing its thing at this time, and I shouldn't be expecting a reading? After it returns readings though, the numbers seem to make sense? There are still a number of 'Malfunction' flags that get returned for various PIDs on NOx2. These screenshots were taken in the Gretio app while driving at about 50mph at 30F ambient air temperature.

I've ordered this NOx2 sensor (part number 24001920). I'll install myself when it gets here next week. Otherwise, we'll keep driving it as normal with the CEL illuminated and see if it decides to go away on its own before then.

 

[Hike208]

I love whales, trees, and bees but damn....why is it so expensive to save those fvcking things?

I love all of those things too, but I also love not getting stranded in the middle of the desert in Nevada because a NOx sensor decided it was going to have a bad day.

The CEL in our truck for P20EE went off on its own. The numbers that I can pull look good, or at least within acceptable range. Things seem to be happy for now, but I'm sure this isn't the end of it for our truck.

 

[Hike208]

Hike208? Any new news?

Heya, sorry for the radio silence!

The short and unfortunate answer is no. 😞

Every few months or so we will get a MIL for P20EE. Sometimes I will clear it manually, other times I'll let it run its course and it'll typically go away within a few longer driving cycles. Funny thing is that Gretio can't clear the code, but a different more generic OBDII app that I use called DaschCommand can clear it.

We have not seen any messages in the DIC about DEF quality or anything else that has triggered the countdown of doom. We make sure to avoid the Platinum DEF.

We haven't done anything to the truck besides change the oil a few times, the fuel filters once, and run a few tanks of DEF since our first incident with the countdown came up. We have not changed any of the NOx sensors since it happened, but I do have NOx2 that's sitting in the truck waiting for our next DIC countdown. As soon as the whole system gets up to normal operating temp, the NOx values appear to be in plausible range.

I am still getting the same faults in Gretio about the NOx 2 heaters and the DEF line heaters. It's starting to get cold where we're at (single digits at night, daytime highs in the upper twenties), and we had some challenges when it started to get this cold last year. The truck lives outside through the whole winter. I believe that this is below the freezing point of DEF, so I'm sure that this isn't helping anything if the line heaters aren't working and feeding the injector.

 

[Hike208]

Following, I’ve got P20EE as well and my dealer just replaced the DEF pump/module which didn’t fix the issue. I’m having them replace NOx Sensor #2 next with fingers crossed. These issue are so expensive.

Sorry to hear that, hopefully your truck is still under warranty and the dealer is taking care of you. Our MIL for P20EE seems to pop up when it's colder outside, under 30 degrees or so. Are you in a colder climate?

Please keep us posted and let us know what your resolution is! Good luck.