June 5

2017 GMC Canyon Denali Diesel NOx Sensor

GMC Trucks


Mark: Hi, it's Mark from Remarkable Speaking. I'm here with Bernie Pawlik, Pawlik Automotive in Vancouver, Vancouver's best auto service experience, 24 time winners, best auto repair in Vancouver, as voted by their customers. And we're talking cars, or in this case, trucks. How you doing, Bernie? 

Bernie: Doing very well.

Mark: So today's victim is a 2017 GMC Canyon Denali diesel, what was going on with this vehicle? 

Bernie: So this vehicle came to our shop with a check engine light and a warning that within a certain amount of time it would shut off and not restart because of a performance issue.

So this is a protective system that many diesels have. If something goes inoperative with the emission system, it will allow a certain amount of driving. Usually the warning will come on 500 kilometres or so before that you have to absolutely fix it. So for instance, if it has a diesel exhaust fluid, which this truck does, if you don't fill the tank, then you know, eventually the warning will come on and the truck will just simply not start. Or it'll run at like 10 kilometres an hour, maximum speed. So you can drive it to the shop to have it fixed. 

Mark: So did you just plug in the scan tool and it told you what to fix? 

Bernie: No, it's a little more complicated than that. But I mean, the first step of the diagnosis is to plug the scan tool and see what kind of trouble codes we have.

So we did that and there was three codes stored. I'll actually just get into the picture portion of this right away. So there's our truck, unfortunately, a little dirty and wet from a rainy Vancouver day. But nonetheless a Denali. It's a beautiful little mini sort of half size pickup truck with a diesel.

2017 GMC Canyon Denali Diesel NOx Sensor

This is the code. Sorry, the picture's not great, it's an actual photograph of the screen of our scan tool, but there's three trouble codes stored. So we just take a minute to look at these. The first code, it says NOx sensor two current performance. 

2017 GMC Canyon Denali Diesel NOx Sensor

So there are two NOx sensors. And what these do is they measure the NOx content of the gas coming into the SCR, this is the rear catalytic converter that deals with the NOx cleanup. So the front sensor will measure the NOx coming in. The rear sensor measures the NOx coming out to see is this catalytic converter working properly? Is the injection of the diesel exhaust fluid correct and so on. 

So sensor number two is giving a performance code. Then the next code says NOx catalyst deficiency below threshold. And then the last code closed loop reductant, that's the diesel exhaust fluid injection control at limit. Flow too low.

So we're thinking, okay, are there three problems here? Is there a bad sensor? Is the catalyst bad? Is the fluid not being injected properly? And so this is where we have to determine, this is where the diagnosis takes on a higher level of work. So we have databases to look at different codes, see what they do, factory information. We look at the diagnostic charts. Look at these items, figure out what it is. 

And so the first place we knew to start with was the actual NOx sensor, it was not performing properly. This code, this P11DC is often caused by a bad NOx sensor. So we replaced the rear NOx sensor. That was the first part of our process. 

Mark: So with three trouble codes, do you just have three things to replace? Or how does this play out? 

Bernie: Yeah, as I was saying, so you could think, yeah, you know, if you go in the theory, oh, you just plug the machine in and just do whatever it is, it seems like there'd be three things that aren't working properly. But everything actually led, and this is where the diagnosis and the skill of what we do as technicians comes into play, is what is causing what to happen. And sometimes you might have like 10 codes and they're all caused by one thing, but you may also have 10 codes and they're caused by three different malfunctions. Or one thing will lead to another. And so especially with a diesel, it's really important to fix things as they occur. 

So the light comes on, you don't wanna wait. Of course this one actually has an urgent message, so you don't have much time. We do run into a lot of diesels where people may have been driving with the check engine light on for a while, and then all of a sudden they've accumulated a bunch of problems because this component isn't functioning.

It soots up this thing. And so after that you're in for like a 5 or $10,000 repair bill. We've seen it on quite a few different diesels. So if you have one, the light's on, get it fixed right away, deal with that and it'll probably save you money.

Anyways, to make a long story short, first thing we start with was the NOx sensor. Cleared the codes and that basically eliminated that performance code. We took it on a road test, but there was still the warning light on for the shutdown of the system. And what had happened, of course, is that there's a reset procedure that required to be done to make sure that everything was reading properly. So the next step in the repair procedure is to go out run a re-gen on the diesel particulate filter.

And then after that there's resetting procedures for the other components. So it's fairly involved. It's not just a matter of we bolt the sensor in and the way you go. There's a number of resetting procedures that take quite a bit of time to do afterwards. So I think I've rambled on a bit there.

2017 GMC Canyon Denali Diesel NOx Sensor

I have actually another picture too. So there's our codes. The other picture is the actual NOx sensor. So this is the actual NOx sensor. It looks a lot like an oxygen sensor. And it actually does have oxygen sensing capabilities. But what's different about this is, this is the actual sensor. 

2017 GMC Canyon Denali Diesel NOx Sensor

I'm moving my mouse pointer around here. This is the actual sensor that screws into the exhaust pipe, from there it actually screws into this little module here, this is all attached. It doesn't come apart. The wiring harness of the vehicle attaches over this point where I'm moving the most pointer.

So there's a whole complete circuit board inside here, full of computer chips and things to analyze the NOx and oxygen content to the exhaust, and then send that signal to the computer. So it's a complex sensor. Much more complex than a normal oxygen sensor, which would have maybe four to six wires that just plugs into the wiring harness.

So that's basically the sensor. As I said, there's two of them on the vehicle. We determined the rear one was bad. Sometimes they both go bad. They are expensive, by the way. But fortunately on this Canyon, it was actually quite cheap. And the whole repair with the diagnostic, replacing the sensor and doing all the relearns is actually the same price as we find on some vehicles, like a Dodge Cummins, for instance. Sometimes these sensors can be over a thousand bucks on certain vehicles. So this one was, you know, the whole repair, labour, taxes and everything in was a little over a thousand dollars.

But it's often cheaper to just to buy the sensor for some vehicles. And they all look the same. So I don't know why they cost more on some than others, but you gotta put the right one in. It has to be the right sensor. 

Mark: So you mentioned that it was a little bit involved in resetting the computers to get everything working properly.

Bernie: Yeah. It's a finicky process of running one reset procedure, then another one, and doing 'em all in the right order. And sometimes it takes us more than a couple of tries to get it all right. It took a little while, but it all got done and worked great afterwards.

Mark: So this is a lot smaller GMC truck. What type of diesel does it have? 

Bernie: Yeah, this uses a 2.8 litre diesel. It's the same type of diesel that they use in the Jeep Liberty vehicles. Same style. Motori, I'm trying to think of the first two. I'm drawing a blank. Must be the grey hairs taking away my memory or something. But anyways, it's an Italian designed diesel. And, you know, it's interesting because, various companies have owned this. I did a little research on it a while ago. I was actually thinking about buying one of these trucks for a very brief moment.

But the actual manufacturer's been owned by a variety of companies, including GM used to own a portion of this company for a while, and then they sold it to someone else. And I'm not sure who owns it now, but they actually make a lot of diesels, a variety of 'em.

I have to say, I'm not overly impressed with the ones that are found in the Liberty, and I believe this engine is pretty much the same design. So they've been actually really reliable in the GMs, but who knows, you know, given the long term, whether they're gonna have the same issues as we found in the Liberties with rocker arms popping apart, and lifter failures and expensive internal engine problems that can usually be fixed on the vehicle.

Mark: Is it a V6? 

Bernie: It's a four cylinder, they're big pistons, 2.8. It actually works well. I mean, it's got lots of torque, power, it does, you know what a diesel does, you know, good towing capacity, good weight capacity for a small truck. So it's the right size for a little truck. So far they've been pretty reliable and good. Time will tell. 

Mark: If you're looking for service for your Denali Canyon GMC or GMC Canyon Denali in Vancouver, the guys to see are Pawlik Automotive. You can book online at pawlikautomotive.com or call them direct, (604) 327-7112 if you're in Vancouver, BC, Canada. You have to call and book ahead. They're always busy. Thanks so much for watching and listening. We really appreciate it and thank you, Bernie.

Bernie: And thank you, Mark, and thanks for watching.

About the author 

Bernie Pawlik

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