The top 5 diesel fault codes, and emissions issues you can do something about

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Updated Jun 21, 2024

At the annual conference of the National Association of Small Trucking Companies a couple weeks back in Nashville, Tennessee, TA Petro Truck Service Vice President Homer Hogg revealed the top five fault codes seen by the three leading all-model diagnostic equipment providers. He analyzed codes data to determine the top five parts of the trucks those codes were related to.

At No. 1 (which might not come as a surprise to any longtime truck owner): Aftertreatment-related codes. 

Howes logoOverdrive Radio's sponsor is Howes, longtime provider of fuel treatments like its Howes Diesel Treat anti-gel to get your ready for winter, likewise its all-weather Diesel Defender, among other products."I do not like this system," Hogg said, referring to the complicated diesel particulate filter (DPF) and selective catalytic reduction (SCR) systems in 2010 and later model engines, built to meet EPA's emissions-reduction target and generally clean up exhaust. While he and a myriad truck owners might long for the old days of less complicated tech, or hope hard for a hydrogen low-emissions solution, Hogg emphasized we'll be living with these emissions-reduction systems for quite a long time. 

Tune into this week's Overdrive Radio episode for an informative look at actionable steps owner-operators and small fleets can take to guard against some of the most common issues seen.

Among the recommendations:

  • When the dashboard lights up, don’t clear those codes. Techs need them to properly diagnose any issue.
  • Clean the diesel exhaust fluid (DEF) doser in the SCR part of the system once a year, at least.
  • Change your DEF filter according to manufacturer-recommended intervals.
  • Keep a “clean room” approach around the DEF tank, particularly when you’re pumping the fluid.
  • Keep SCR-system efficiency tests in mind to periodically examine it to prevent expensive NOx sensor failures. Knowledgable shop techs can perform these tests, as can any fleet whose made the investment in the right equipment and technician training

Overdrive Radio logoSubscribe to the podcast on your listening platform of choice for early access to the weekly Overdrive Radio series -- it drops typically every Friday to the feed and follows here at and in Overdrive's Youtube and Facebook feeds the following week. You can subscribe for first access via Apple and Google podcasts, Spotify, TuneIn, most anywhere you listen.Most importantly, perhaps, don’t fall for the emissions "delete kit" trap, if you want qualified mechanics to be able to help you work issues out. The result can be devastating if the Environmental Protection Agency and its enforcers come calling, to both your own and any associated shop's business. Hogg said a friend who just so happened to be sitting at a dealer shop east of Nashville the morning of his presentation was told by the shop the issues he was experiencing were emissions-related, yet: the shop could do nothing about it without first bringing the truck back into compliance. The owner, Hogg said, had fallen into the trap and had an emissions delete system in the truck. Take a listen:

Other potentially useful past emissions-related coverage in Overdrive:

Partner Insights
Information to advance your business from industry suppliers
The ALL NEW Rand Tablet
Presented by Rand McNally
BlueParrott B350-XT
Presented by BlueParrott

**Diesel emissions problems; Getting the right diagnosis, and the right repair, on the first shop visit 
**Delete emissions problems without deleting the system
**Fuel treatments increasingly part of owner-operators' PM routine to ward off emissions issues

[Related: Emissions 'delete' scheme leads to max penalty for fleet, shop]


Todd Dills: Hey everybody. Todd Dills here, your host as usual for this Overdrive Radio podcast, dropping to the feed November 17th, 2023, and then at the world-famous, the 20th.

This one features an informative look at persnickety emission systems in 2010 and later trucks, with new actionable steps owner-operators and small fleets can take to guard against some of the very most common issues seen.

The primary voice here? It's none other than TA Petro Truck Service Vice President Homer Hogg, who presented this talk at the National Association of Small Trucking Companies’ annual conference a couple weeks back. Hogg surveyed the three leading all model diagnostic equipment providers, and asked for data on fault codes seen over the course of recent history and analyzed those codes to determine the five biggest parts of the trucks those codes were related to. Here's Homer Hogg setting the stage for the results.

Homer Hogg: Aftertreatment is number one, and I'll prove that here in a minute. I'm going to give you an overview in the operation of the system that's given us the most trouble, where you are likely spending the most money right now. And then I'm going to give you some diagnostic steps, and hopefully before we get out of here we'll have enough time, because I have a slide that I entitled Do This Right Now. So what should you be doing, if I were you? What would I be doing when I went home? Maybe even before I got home?

Todd Dills: Yeah, that's right. Aftertreatment is number one, as you guessed it. And as you'll hear throughout the presentation, it's number one by a long shot. Any wonder the popularity of so-called “emissions delete” kits? A member of the audience asked about such things near the end of the presentation and Homer Hogg's response to the question though bears emphasizing here up top for anyone in the audience who may have pondered, or been approached about, such a delete kit.

Homer Hogg: Listen, we're going to run out of time, but I have a lot I can say about that. Do not do it. Okay, I've got three minutes. I've got a friend of mine right now sitting at a dealer here locally, it's ironic in Nashville, Tennessee. He's a little east of here. He's sitting in a dealership. He's got an engine problem and they're telling him it's some aftertreatment stuff. They found out he's got a delete kit, and they told him, “come tow this truck” out of their shop.

And he says, "But I need it fixed." They said, "We'll be glad to fix it, but number one is we're going to get your truck back into compliance." The EPA will issue you a fine, and they are not ashamed to do it. Not only will they issue you a fine, they'll issue a fine to that servicing network, and they could issue one to that technician as well. And so we are very clear in our network to our technicians, "If you want to work here, don't do that, right, because we're not going to take your truck out of... We're not going to move your truck into a violation position, to protect you and to protect us." So I will tell you, I will never advocate, as much as... Now for the record, I do not like this system. The engine runs much better without it. But you can't hardly do it today. It's illegal, it's putting too much at risk, and it's also putting your engine at risk in many ways, because if you don't do that lead kit right you'll do more harm than good.

Todd Dills: On the other side of a break for a word from Overdrive Radio’s sponsor, we'll jump right in with TA Petro Truck Service Vice President Homer Hogg about what he does recommend owner-operators do to better delete emission system issues, without deleting the system itself. Keep tuned.

Speaker 3: Stop fuel from gelling this winter with Howes Diesel Treat, North America's number one trusted anti-gel. Right now you need Howes Diesel Treat more than ever, not only to keep you gel-free, but to fight the shortcomings of today's ULSD by adding vital lubricity, removing water, and preventing deposits. The only guaranteed anti-gel on the market, Diesel Treat also boosts fuel economy and improves performance. This winter do yourself a favor and add Howes Diesel Treat at every fill-up. Visit for more information.

Todd Dills: That's H-O-W-E-S, Now here's Homer Hogg setting us up.

Homer Hogg: So here's the data, you might want to take a picture of this. If you want to come up and get closer, you can. I'll walk you through this. So the top five are going to be on your far right, the one that covers the most surface area is going to be emissions. I have 57%, I think that number is larger, because I took out some of the noise, because I just didn't have time to dig into those that were on the edge. I just couldn't conclude what those codes were.

I can tell you I think that number's closer to 60%. So what does that mean? That means 60% of the codes that are causing your lights on your dash to come on are emissions related. After that you have electrical and lights, believe it or not, but it's so far down the list there's a huge difference between 57% and 15%. But number two on the list was electrical and lights.

Number three was radar systems. Your trucks are starting to get more and more radar type devices on them. And then we have engine severe. About 4% of them are codes that are sometimes manufacturer codes specific, but they say engine severe, and I won't get into those today. We don't really have time for that.

And then finally the last 2% in the top five were HVAC, which I found to be fascinating, to tell you the truth. We didn't even look at your models, it's just in the last year trucks that had the light come on. We didn't discriminate against any brand, any age either. I have all kinds of data. This is just, okay, I got 40 minutes. I got to say, "Okay, where I did get the value out of these 40 minutes?" And I'm going to hit you with it best I can.

So I've got some good news, and I've got some not so good news. The good news is the majority of the codes were similar to this, right? And so this SPN 521049 with [inaudible 00:06:42] 31, that is very, very common code. That's only one manufacturer's way of presenting that code, but it doesn't matter the manufacturer, they all have similar type codes. That's not even that important right now. What is important with this slide is about three things. So here we go. I'm going to read what's in the top box. Now I took this right out of the service software, so on the software it gives you diagnostic steps based on the codes.

So, I went into the software and just took an extraction out of there. And I'm going to read it to you. Are you ready for this? It says this code sets when there is a code present for diesel exhaust fluid, DEF, tank level, DEF dosing unit, DEF pump, NOx sensors, blocked DEF line, DEF quality, NOx efficiency, selected catalyst reduction, SCR harness and RSCR sensor.

Now imagine that you have this work order and somebody has sent you out to fix this trunk, you're looking at this manufacturer like you lost your mind, right? So you're telling me this code comes on when anything happens in the aftertreatment system? Yes, that's what happens.

So we have to be able to narrow this down, and get a little more information. So here's my point here, the code only gives you limited information on where to start this process. So if you have technicians that are working on this, you can't say, "Here's the code, go fix the truck." Because they're going to get this stuff right here. Here's what else I want you to understand, at some point in time if you don't respond to this, and if you have drivers you want to have this dialogue with them, at some point in time and I know you're going to ask me, "Well, when?" I don't know when, it depends on the age of the truck, it depends on the severity of the code. But at some point in time, if you don't do something about this, you're going to get a 25% D rate.

And if you don't respond to that, you could get a more aggressive D rate. And if you don't respond to that, you could get damage to components, or you're probably going to get a shut-down. But here's the deal, every time it goes to the next level, it costs you more, in most cases. So what's my point? You can't let these lights stay on and not react to them. Time is not on your side.

And so what do you do? I'm glad you asking me. At the bottom left, everybody see what it says? It says Verification. So the manufacturer is saying, "Here's what you do." You do a part SCR efficiency test, for extended SCR faults, you do a high idle regeneration, if you have a DPF or NOx fault. So don't worry about that, but here's what I need you to be concerned about. There are two tests you have to run. You're not really going to be able to run those without diagnostic software properly, and a technician that knows how to use it. Okay?

And so, when you get the majority of the lights, you're going to need some assistance. What's the note at the bottom say, anybody read that?

Speaker 5: Do not clear the codes.

Homer Hogg: Do not clear the codes. And you're going to see in a second why. Because the tendency of a lot of people is, "I can know how to make it go away. I clear the codes and then perhaps..." I know somebody's laughing, but I know how many you guys do that because I see us all the time, because it comes in and somebody's already cleared the codes, and now they say, "We want you to fix the truck." And I'm [inaudible 00:10:24] to say, "What happened to the codes and I'm going to show you why I need them." Do not clear those codes. We need that info to properly diagnose the problem. Why? If no other reason because of this box right here, it could be anything that's in the system. So, we use the additional codes to understand and chart the correct path that we have to take. If not, we're just flipping a coin and guessing where to go.

All right, let's dig into this everybody. Okay? All right, you guys ready for a headache? You can come on up if you like. All right, here's what you're up against. Here's the system. I don't care whose truck you have, it's going to be pretty similar to this route. So on the left-hand side, you've got an engine, at the very top, the blue stuff up there that is your charge air cooler. Are you with me? Everybody good? Obviously we see the turbo on the right, this thing in the center is [inaudible 00:11:22] thing right here is going to be your diesel particular filter. Everybody loves that DPF and a regen, right? Anybody in here love a regen? That's what I figured. Hardly nobody.

It costs time, it costs money. Okay, you got your DPF, and then when you get down here to the end, you've got another funny look at acronym called a SCR.

That's your selective catalyst reduction system, or that's why we have DEF on a truck. All right? Let me show you how this works. So when your engine is running, you've got fuel and air in there, you're creating what we used to know as black soot, right? We used to love that stuff going down the road. I'm an old dude, I don't look old, do I Lise? Don't answer that. I don't look at old, but I remember the black soot, and we used to love that stuff, right? In fact, we would do things intentionally to get more of it. Don't tell the EPA, anybody from EPA in here? Might be, I better watch what I say. But we used to do that intentionally and let that black smoke... And in fact if we were really good, we got flames to come out of those pipes as well. We don't do that stuff anymore, by the way. Don't do that.

But your engine doesn't know it's a new engine, so inside of that engine it's still creating that black soot. It hasn't gone anywhere. It's the same diesel fuel, well not really the same diesel fuel, but it's still burning diesel fuel, and it's still generating black soot and smoke, but there's a way that we're preventing that smoke from hitting the atmosphere. So, you create that black smoke in that engine, you also create something else called oxides of nitrogen. NOx is what most people know it as, NOx. And so that's the bad stuff. If you ever come across a hill in LA, and you start to go down, what do you see? That's right. Nothing. Because you can't see the city a lot of the times, I've been across that hill many times, and when you drop down off in there, I know there's a city down there somewhere, because you're seeing smog and a lot of that are oxides of nitrogen, not good stuff.

And so we got to treat both of those things. So you're with me? In the aftertreatment we're trying to deal with two things. Particulate matter, that's black soot. Oxides and nitrogen, that's the bad stuff in the air that's really causing a lot of health problems, okay? We've got to treat that, and we have legal reasons to treat it because the government requires us to do that.

So, we're going to treat the black soot in this device right here, and in this system. This is our DPS system, diesel particular. So we go in there with the black soot, and we go in there with the NOx. And really all you have on the front of of it, if you were open this up, it looks like a big honeycomb in there. It's just like a catalytic converter on your car. There's a big old ugly looking thing that's got precious metals on it, and when you put... In fact this thing, I love this thing right here, by the way, this dumpster, because all this is Dave, is a way for us to inject raw diesel fuel right into the exhaust, it makes a flame-thrower. I wish we could open that up and drive that truck down the road like that.

Don't do that by the way, because you're going to burn your truck down, so don't say that that Homer Hogg at TA told me to do that, but that would be interesting to see what that looked like on the track somewhere. But all we're doing is injecting raw diesel into a hot exhaust to raise the temperature up, so we can take any leftover fuel, which is black smoke, and turn it to ash, right? That's all we're really trying to do. We do that in the front with this fun looking device called a DOC.

After it leaves that it should be gray looking ash that's much smaller, and we trap it in something called a DPF. Just think of a big old brick that's got holes drilled in it, so the smoke goes in, goes through the sides of the brick, the brick traps it, and it can't come out the other side because it's not open. So, it doesn't let the particular matter come out. That's all that's going on in there, that thing's given us fits. Anybody had to do a regen in the last six months? Raise your hand. Have any of your drivers had to do a regen the last six months? Probably most of you, most likely. Anybody who's driving local, you're driving heavy loads, you're in a lot of mountain terrains, you're probably regenning and hopefully you don't cuss, but if you do, you're probably doing a lot of it, okay?

Because you're probably going to have to stop your truck, you got to make a load and you're having to wait, right? You don't really have a choice, not really. And so then we do it, we do our thing, get rid of the black smoke, but we still got to deal with the NOx. So we leave there, and then we go down into this next system. Here's the tank you're putting DEF in, and DEF is urea, we're going to talk about that in a minute. So now we got to treat NOx. So we go down, we inject DEF into the system, that all of that now goes into the secondary catalyst system, the selective catalyst system. I'm not get too much into chemistry, but inside of there we've got the urea, we've got the oxides of nitrogen, and we've got another catalyst in there. It heats all that up and chemically it starts separating it, and when we finish coming out of the exhaust, you've got water and you've got nitrogen. Harmless, nothing to it.

That's what comes out of the exhaust. Or let's say that's what should come out of the exhaust. That's not happening, which is why the light's coming on. Are you with me? So, couple reasons why your light's coming on. We're going to get in detail here in a minute. But either the sensors that are over here in this DPF, they don't like what they see, or the sensors that are here in the secondary catalyst, they don't like what they see. These doggone sensors. When this system is working properly, we have taken these trucks, put it in a room, like we've done it in hotels, you're not supposed to do that so don't tell nobody, we've done it in hotel rooms... Not a hotel room. But in the convention centers, they got these rooms, we put trucks in there, close the door and start, and just let them run. Not an issue.

Don't do that, okay? And don't tell somebody I told you to go do that. But to Dave's point, what's coming out of the exhaust, if it's working properly, is much cleaner than anything we've ever seen. So, this particular system, by the way, does work. But it's some timing, it's got a mind of its own and it's when it's not working when we have a problem. So, right now today for sure in the last 12 months, 57% of the time now those lights come on your dash, it's because something went awry in the system. And so, I know what it is and I'm going to help you with it. So my job today is to help you keep your truck moving, and what can you do right now? [inaudible 00:18:22] move it up and down the highway. Diesel exhaust fluid, I say by now most people know what it is. In fact about 32.5% or so of urea, ammonia nitrate, right, I'm old farm boy. So we used to have it in little white beads and drop them, we had to use the old school spreaders that dropped it down the bottom. So we dropped a lot of ammonia nitrate on fields. Mix that with water and it's a clear liquid, it's non-toxic, it's also biodegradable. If you're interested in how much it weighs, fairly heavy, 9.2 pounds a gallon.

Here's very important, it will freeze at 12 degrees. In fact, there's a heater system built into it that we have to maintain. Sometimes it's a problem, we've got coolant coming into that system through a tube system to try to keep it thawed out and keep it at the right temperature, but it will freeze on you. Don't worry about it. It doesn't hurt it to freeze, right? It'll thaw out and go right back to working, but it will freeze on you, so that's something we have to be aware of.

I'm not going to get too much in this, the pH levels, I thought I'd do something kind of fun, and Coca-Cola is pretty acidic. It's sitting down here about two, three on the pH scale. DEF is sitting over here, it's alkaline. See, what's the difference? Well, if you slept like a lot of us did in chemistry class, you don't really care, right? Because you had something else on your mind, but when you're in chemistry class, they look at that opposite poles of that, acidics and alkaline. Why do I need to know that? Well, the reason you need to know is right here. Acidic corrosion can result in rusting. Alkaline corrosion will give you pitting.

So I can look at the type of damage and tell what's on that metal. You see, it's important to note if you've got metal issues, chrome issues, you need to figure out where that's coming from. If you've got some vapor coming out of one of your lines, or somewhere out of your aftertreatment system, you need to know what's causing it, but we can look at the metal and if we know what we're doing, we can pretty much tell you where it's coming from. So that might help you down the road.

Okay, so let's get into, I'm going to spend the rest of my time talking about what we can do. What can you do that's going to help you? So if you're not familiar with these, DEF quality is one of the main reasons why that light comes on. It's because at the end of that SCR, there's a NOx sensor, an outlet NOx sensor. Now we have to determine whether the sensor's bad, which by the way, Dave, sometimes they are. Those sensors are in a very unfriendly environment, and I'm telling you, you're going to see on my last slide, they are not cheap, by the way. And so, it can be a sensor, or it could be the sensor's telling you the truth.

If the sensor's sending information back to the aftertreatment computer, and by the way the truck has an ACM on it, and that's an aftertreatment computer, it's got its own computer to manage that system, that aftertreatment system, to look at the information coming over on those sensors, to look at the pressure information that's coming over. It's managing the pump, it's doing a lot of things, but there's a computer in there.

That computer is taking information, and then it's communicating that to the operator of that vehicle. Are you with me? Everybody good with that? All right, so you've got DEF quality issues that we are seeing, and we're seeing quite a few of them. Let me ask you a question. Anybody have a gallon of DEF in a truck right now? Raise your hand. Anybody got a gallon of DEF? Got a gallon at home, in jugs, five gallon jugs? I love this [inaudible 00:22:00] because let me tell you about those gallons... And by the way, I'm finding them in trucks, so I'm glad it's not in any of your trucks. We are finding gallon jugs in trucks, and they've been in there over a year, and they're wondering why they're getting DEF quality lights. This stuff has a shelf life.

It's not going to survive in your cargo box in Dallas, Texas in the middle of summer, and expect a year later it's going to be effective. It's just not going to be. Okay. It's got DEF and the urea in there, it's got water in there. Guess which one's going to evaporate first, right? So now your percentage is off, and that NOx sensor, which I don't like that joker, but that NOx sensor at the end of that SCR knows the difference. So it's got to take a reading, and by the way, that NOx sensor has to be calibrated to the EPA standard. Are you with me?

Not only to the manufacturer, and what the manufacturer needs to say that system is operating properly, but it also has to be calibrated to what the EPA requires as emission levels coming out. Everybody with me on that? Okay, so it could be when that sensor is telling the computer I have a problem, that you've got a quality problem. Dave [inaudible 00:23:19] talked about this a little bit today, but there's something called ammonia slip. All that simply means, when we say a urea, we're really talking about, let's just call it ammonia for now, or it turns into ammonia. If ammonia gets through that SCR device, that sensor picks it up, we call it slip.

It's slipping through. That shouldn't happen, right? So ammonia slip is a problem. Water poisoning is a problem. If you get too much water vapor in that system, the water vapor will actually damage the sensor. Your NOx sensor, it'll damage your NOx sensor. And so you'll end up with sensor damage. You've got a filter in there. I'm shocked. This happens to us regularly in our network, when we're doing a PMI on a truck and we ask the customer about, "Do you need your DEF filter changed?" They said, "What DEF filter?"

How long have you had that truck? They don't even realize there's a DEF filter on it. It's out of sight and it can be out mind. So if you don't know you've got a DEF filter, you need to figure that out. And by the way, I would look... So I know what you're going to ask me, right? "Well, when do I need to change it?" I see service manuals, and by that I mean service literature, because we don't use manuals anymore, that go anywhere from 200,000 all the way up to 500,000, and everything in between. So the manufacturers haven't agreed upon it. So you need to look at your specific truck and determine what the manufacturer's telling you you need to do with your filter and get it changed on time, it's going to help you. Foreign liquids in the exhaust, I won't tell you everything that we found in DEF tanks.

Some are just disgusting, right? I'm just telling you, we find everything. We have test kits for the DEF, you don't want contamination in that tank, right? We want what's supposed to be in there. We want water, and we want product, right? Urea. So, foreign liquids in the exhaust. But here's what we see mostly. We see internal oil, coolant and fuel leaks. So let's stop right here, this is going to save you a lot of dollars, a lot of time. If you are noticing that you have coolant disappearing, but you don't have external coolant leaks, you need to get in a hurry and find those, because if it's an internal leak, and it's going into your exhaust system, and it makes its way into the exhaust system, bad, bad things happen.

Those things cost you five zeros, at least, maybe more, to get those things fixed. You do not want coolant hitting or oil, or fuel, hitting your DPF, and god forbid it slips by that, and gets into your SCR. So we've got to find leaks. We can't do what we did in the old days, we just keep topping off and rolling, right? I'm sorry, don't do that. Find you someplace to get that corrected.

All right. So I just... I'm not getting a little too deep here, if I do just throw something at me, but I want to show you what this looks like for real. Okay. So what you have here on the far left is a dirty NOx sensor, right? Are you with me? Our injector, NOx-doser. We'll get to the sensor in a minute. A dirty NOx-doser, in the middle is a clean one. I want you to absorb that for a minute. Okay? The one on the left, it's got... You can't really see it maybe at a distance, but it's got white powder on it, built up, it's crystallizing. That happens whether you want it to or not. So the manufacturer will tell you when the light comes on and you get certain codes, you need to have that thing pulled out and clean. Here's what I'm going to tell you, you need to figure out on your truck and your operation how you need to maintain that.

If you want to save yourself some trouble, do this intentionally, bring it into your shop once a year, my bet for you is twice a year if you're having this problem, have somebody pull this thing out and clean it, and stick it back in. Clean it, stick it back in. You're going to be amazed at how few check engine lights you're going to see if you do that. I'm not charging you for that, Dave. I don't know if you're going to charge them for it, but I'm not charging you for that, but you're going to find that to be worth something to you. Just pull it out, inspect it, clean it. It should be clean, because it has to not only inject, but it has to inject at the right pattern.

It is not enough for it just to be injecting, it has to be able to get the right pattern so that it get the right type of vapor getting into the system. And this is what it looks like in the pipe, because you won't see this, right? You see this kind of ugly look at stuff, I use a soft brass brush is what I use. I don't like salt, right? Yeah. Sometimes I might use some spray cleaner of some kind, but usually a soft brush and a rag, you just want to get that stuff off there, and a brush does a really good job. Soft brush does a good job.

Now when you're cleaning it, I caught a guy doing this one day, he was holding it over his fuel tank. What are you going to see in just a few months? You'll see pitting, right? And you'll wonder where that's coming from. It's because you've got to driver out there cleaning that thing, he's smart enough to know I need to take it out and clean it, but it's falling on top of the tank.

Okay. So to do diagnostics, we have to do a parked SCR efficiency test. We have to do a high idle regen. Remember that top right box where we had all of those things that could be wrong. How we isolate the real problem is by performing these tests. Those tests will tell us what's wrong and what's not, right? Which component's still functioning properly and which one I need as a technician to go chase. Are you with me? So we don't chase ghosts. We run these two tests, and I'm going to tell you right now, good techs going to be able to tell you. For example, if you run that efficiency test, somebody says, "Well, give me the number." Okay, you want a number? I'll give you one. It should be 95%. On a new truck it's going to be like 95%, or better. But what's good, or what's bad?

It depends. If you have a 2010 EPA package, then you're talking about 80%, or higher. If you're talking about 2014, and depending on the truck, you're talking about 85% or higher. So, I can't give you... It's not that clear cut. But just know if it's functioning properly and everything's good, you're in the high nineties. Your efficiency, it ought to be in the high nineties. Now, when does it fail? It depends on the year. It depends on the manufacturer. Does that make sense? Okay.

I wish I could give you a clear cut number. I'm just trying to give you some guidelines to work by. But I think we're going to live with this for a while. Anyone bought a truck in the last five years? Yep. So you're going to live with it a long time. Last three years? You sure enough going to live with it for a little while. And so, we're going to have it for a few years to come I think.

I pulled this right out of a book. I went into the Detroit Service Information Bulletin that one of our techs would use if they had this problem. Okay? This is the exact first part of the procedure. So what does it tell you? It says, "Check as follows." It says, "Connect diagnostic equipment." Well, okay, so we connect our tools. So if you don't have that, I'm not sure what you're going to be able to do. You got to have some kind of diagnostic equipment. You got to have that, "Turn ignition key on, keep the engine turned off." So we're not going to start the engine, "Check the ACM software level."

So we're going to have to find out what the aftertreatment computer has as the level of software, and compare that to what the manufacturer says they should have. So by the way, most of the time when we're doing this, we're live online, so we're hooked on wireless so that we are bouncing against some database that gives us the most recent version of that software. That make sense? Most of the time. If you're using online service literature, it usually has it right in there. The service literature will say, "You should have 1.01.923.3007." And you just compare your software with that. And if it's out of date, a lot of times you just do the update, light goes out.

Oftentimes, if you do that update, you can go on down the big road and keep on trucking. So, we got to find that out right away. If it is, we go right to step number four. Number four says, "Check for other emission related fault codes." Here we go. We were asked that question earlier. Now the technician is told by the engineers, "Are there any other emission related codes?" Here's why they're asking the tech to do that. Are there any selective catalyst reduction, or DPF related fault codes present? If there are, go fix those. That's how we prioritize.

And what we will do is based on count. If the computer has seen one of those 50 times, and one of them two, guess which one we're going to go chase? Okay? Does that make sense? You got to use logic as you go through there. But that particular portion of the diagnostic routine will typically solve your problem. So, if I were you, what would I do right now? A few things.

Speaker 6: Sell the truck.

Homer Hogg: What was that? Sell the truck?

Well, I'm not going to say I wouldn't have a pre-2004 emission package. I might, right? But that's just me. But get your DEF doser serviced at least annually. So learn how to do it yourself. Get somebody to do it. It's not that difficult. The hardest part is once you do it a time or so, then it comes apart easier. So once you get accustomed to it, there's not that much to it. Get it cleaned. Get it put back in. That alone will save you a lot of downtime.

Change your DEF when your manufacturer tells you to. I would like to give you a time, but I can't because it is raw, we have to look it up every time when you come into our network. But change your DEF filter, and if yours has not been changed, listen, I'm telling you, not only does it turn your light on, but it starves your pump. Okay? Are you with me on this? So when you starve a pump over time, it will damage that pump, so we don't want that pump to be starved, okay?

Train drivers to maintain a clean room of sort for DEF. I'm going to be as nice as I can about this, but there are certain things that don't go in that tank. So we have to have a clean room mentality around that DEF tank. So before we take the cap off, what's around the cap? Is there dirt built up? Is there snow and ice built up? We can't just quickly reach in there and take that cap off and get to putting DEF in there. We have to have the mentality that I only want good, current, quality DEF in that tank. It'll save you a lot of time and trouble. And I guess I just might as well go ahead and say it, because I just get myself in trouble all the time. Please, let's not find urine in the tank anymore. Okay?

I'm just going to leave it at that. Okay? And you can say, "Well, how often do you find that?" Way more than we should. Okay. So let's keep these tanks clean. Dirt, water, snow and ice that falls in there, we have to stop those kinds of things, right? And certainly, unfortunately, find a lot of diesel in there. You can imagine. We have found gasoline, of course, when you find gasoline, you don't have to worry. The truck didn't go very far.

Okay? NOx sensors are expensive. Several hundred bucks if you buy them locally, and you've got a really good deal. Imagine what that same NOx sensor costs 1500 miles away in the middle of nowhere, in the middle of the night, okay? So, they are very expensive normally. If the unit breaks down, you're probably going to have a tow. You're probably going to have up charges that you shouldn't be trying to put in your maintenance cost.

So run a NOx sensor efficiency test and use those results to determine what to do. A good technician, take it to you... You might say, "I don't want to do this." Schedule it into your dealer somewhere, schedule it into a good shop that you trust, and let them run this test and let them review the results with you. You'd be shocked what you'll learn, and they'll give you issues that are about to occur that'll keep you from breaking down.

Learn to do it yourself, if you've got diagnostic equipment, get better at this. I would, if you tell me there's 60% of where I'm spending my money is in this area, I'm telling you I'm about to get smarter about this. We do this every day. There are many fleets that figure this out, and that truck can't get back in their shop, they bring it to us. Because they know we know how to do this and it's not... I would like to say it's rocket science, it's really not. To do this test takes maybe 15 minutes. If we got to go into the DF side, then you know some DF test going to run 30, the regen is going to run 30, 45 minutes. But usually to do any of the repairs, an hour, right? To do anything, we got to take the NOx sensor out. Got to let it cool.

Todd Dills: And so for a bit of a recap, when the dashboard lights up, don't clear those codes. Clean that DEF doser once a year at least. Change your DEF filter when your manufacturer tells you to. Keep on a clean room approach around the DEF tank, particularly when you're pumping it in.

Keep efficiency tests in mind to periodically examine the system, and don't delete it, if you want qualified mechanics to be able to help you work on it. Big thanks to Homer Hogg for walking us through it, and Dynastic, our Small Fleet Champ Program sponsor for hosting him this year at the annual conference here in Nashville. I'll post links to other emission systems, tips we've compiled through the years in the show notes, wherever you're listening. Overdrive Radio is on Spotify, SoundCloud, Apple and Google Podcast. Tune in most any platform. Subscribe so you don't miss an episode. And if you're enjoying these, leave us a rating or review there. Appreciate that for sure.