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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 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 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.

I also have no idea whether GM coded hooks to detect the pulled connector.

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

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Discussion Starter · #22 ·
@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!
 

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



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.



I'll be sure to report back on whatever happens!
The end of line termination will be in the electronics box on the frame rail. Just a heater and a couple grids in the sensor.
The only connection to pull is the chassis harness at the electronics box.
The sensor is hard wired to the electronics box. If you disconnect either end of that, you have broken something. The termination to the box looks a little connector-ish but it is actually a hard wired strain relief.
 

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2016 CCLB Mini-Max Diesel
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Discussion Starter · #25 ·
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.

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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.

View attachment 421882 View attachment 421883 View attachment 421884 View attachment 421885
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.
 

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Discussion Starter · #27 ·
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.

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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.

View attachment 421887
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.
 

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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.

View attachment 421887
As others have said I would suspect the Nox2 sensor more now since you are getting malfunctions on the heater for it. 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. The electrical module connected to the sensor probe does the sensor signal conversion. No analog sensor signals on this go back to the ECM. Heater problems will also set DTCs and if you have the ability with your scanner they can be tested manually using the OBD II connection.
 

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Discussion Starter · #30 · (Edited)
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!
 

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Discussion Starter · #31 ·
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.
 

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Discussion Starter · #32 ·
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.


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Similar boat.

50k miles - Bad Def Fluid warning...cleared after 40 miles.
138k miles - DPF cleaned and GDE tune installed
140k miles - NOX sensor CEL (can't remember the code right now) en route CO to TX. Cleared code, kept driving. Did the timing belt and headed back up to CO a few days later (yesterday). Added 2.5 of DEF that was in my closet for 6 months.
I get within 250 miles from home.... 2am....sleeting snow blowing in NW Texas cow country and the same CEL pops. 5 min later, the dreaded "99 miles to 65mph" message. I stop at Loves to nap. This morning I top off with fresh Loves DEF, top off fuel and start driving...coordinating my distance to home with a buddy on the phone. Maps...protractors....sextant.... gps...calculator.... "I can make it to this point...meet me there with the trailer"...etc.
65mph threshold....clear the Raton pass...eyes set for Walsenberg..maybe Pueblo. Miles left to 55mph.....5, 4, 3 ,2 poof....everything clears...I finish the drive home at 75mph.
Damn gremlins....earlier DEF I added was bad and the new Loves DEF mixed enough to make everything happy....who knows.
Ordered new NOX 1 and 2 sensors and a new DPF injector... I drive too much to wait for stuff to crap out. Will replace items myself then drive to the local dealer to reser, relearn, or do whatever they need to do after sensor replacement.
Talking to a local shop next week to make a custom 3" turbo back and hopefully do the snippy snippy in summer. F U EPA.
I love whales, trees, and bees but damn....why is it so expensive to save those fvcking things?
 

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Discussion Starter · #34 ·
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.
 

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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? Any new news? I'm still dealing with this issue starting in July. Our screenshots with the Gretio app are pretty spot on. But my NOx Catalyst Efficiency is reading -2.00, where I think before it would read 0.00.
This first started on a road trip. Error came on so googled it, tried putting more def in. Didn't clear at first but eventually did. Then came back...So drained it in an AutoZone parking lot and put new stuff in. Didn't clear and kept driving. Read more and tried replacing NOx sensor 2 I got from dealer. But didn't know about the service bay tests to actually clear it. So towed it home a few days later. Bought a module, bought Gretio, performed tests, message cleared. But like Hike208, different circuit tests were failing, mostly for NOx 2 and heaters. After error came back, took it to dealer. Told dealer what I did and they basically repeated it for lots of moola. Drained DEF tank, ran tests, saw failing sensors for NOx 2, replaced sensor I had replaced. Error cleared but not sure if these tests ever passed. They did also inspect the system and found a cracked DEF injector and replaced. But the whole thing cost me ~$1900 for them to pretty much repeat what I had already done. And apparently the cracked injector wasn't the issue because after two days error is back. Just ran a test and again getting random failures like I always have. NOx 1 and 2 numbers seem to make sense. But just noticed the -2.00 catalyst efficiency. Truck has code P20EE. So maybe the catalyst is bad? I swear I'm about to lose it with this truck. Like wtrbrdm said, I'm all good with clean diesel, prefer it. But for $800 I can order a system and tune to get rid of it and never have to worry again. Plus GM has NO idea what's going on? What's that all about? Someone please help!
 

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Hike208? Any new news? I'm still dealing with this issue starting in July. Our screenshots with the Gretio app are pretty spot on. But my NOx Catalyst Efficiency is reading -2.00, where I think before it would read 0.00.
This first started on a road trip. Error came on so googled it, tried putting more def in. Didn't clear at first but eventually did. Then came back...So drained it in an AutoZone parking lot and put new stuff in. Didn't clear and kept driving. Read more and tried replacing NOx sensor 2 I got from dealer. But didn't know about the service bay tests to actually clear it. So towed it home a few days later. Bought a module, bought Gretio, performed tests, message cleared. But like Hike208, different circuit tests were failing, mostly for NOx 2 and heaters. After error came back, took it to dealer. Told dealer what I did and they basically repeated it for lots of moola. Drained DEF tank, ran tests, saw failing sensors for NOx 2, replaced sensor I had replaced. Error cleared but not sure if these tests ever passed. They did also inspect the system and found a cracked DEF injector and replaced. But the whole thing cost me ~$1900 for them to pretty much repeat what I had already done. And apparently the cracked injector wasn't the issue because after two days error is back. Just ran a test and again getting random failures like I always have. NOx 1 and 2 numbers seem to make sense. But just noticed the -2.00 catalyst efficiency. Truck has code P20EE. So maybe the catalyst is bad? I swear I'm about to lose it with this truck. Like wtrbrdm said, I'm all good with clean diesel, prefer it. But for $800 I can order a system and tune to get rid of it and never have to worry again. Plus GM has NO idea what's going on? What's that all about? Someone please help!
You are not running platinum DEF right, That's a no no. Have you been checking DEF manufacturing date too before putting it in. DEF has a max shelf life of 2 years in ideal conditions (not in sunlight and I think around 70 degrees). Heat and sunlight decrease shelf life. Sounds like to me you need to check the power wires supplying to the sensor : violet/light blue and black (2016 colors). If it was a Canbus problem would it would cause problems with NOX1 and particulate matter sensor too. Dealer don't typically like to electrically troubleshoot takes too much time with flat rate. They would rather throw parts at it before doing that.
 

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You are not running platinum DEF right, That's a no no. Have you been checking DEF manufacturing date too before putting it in. DEF has a max shelf life of 2 years in ideal conditions (not in sunlight and I think around 70 degrees). Heat and sunlight decrease shelf life. Sounds like to me you need to check the power wires supplying to the sensor : violet/light blue and black (2016 colors). If it was a Canbus problem would it would cause problems with NOX1 and particulate matter sensor too. Dealer don't typically like to electrically troubleshoot takes too much time with flat rate. They would rather throw parts at it before doing that.
No sir. Haven't purchased DEF in the two years I've owned it. Always filled up at the truck stop.
 

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Discussion Starter · #38 ·
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.
 
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