Top 10 Facts About Diesel Exhaust Fluid

Diesel Exhaust Fluid Container

Photo by Bob Carpenter

By Bob Carpenter

For many years, owners of three-quarter and one-ton pickup trucks enjoyed the fact that there was no smog equipment on their trucks. But in 2008 the EPA required diesel particulate filters on all three-quarter-ton and larger trucks, and required biannual smog tests that included a thorough visual inspection to make sure all the parts were still on the truck. In 2010 the regulations got tighter.

Related: What's the Best One-Ton Heavy-Duty Truck for 2018?

Smog equipment on heavy-duty pickups was thought by many to signal the end of big power and torque; those people swore they'd never buy a new truck again. However, a lot of those same folks have learned you should never say never. You see, a funny thing happened on the way to meeting smog regulations: Every one of the manufacturers figured out how to cut down the bad nitrogen oxide levels while making more power and torque than ever before.

One key to this work around was the use of selective catalytic reduction. Most of these systems use diesel exhaust fluid (a mixture of urea and deionized water) sprayed into the exhaust stream to break down the generated NOx into harmless nitrogen and water. But the fact that the fluid is introduced in the exhaust (often called after-treatment technology) leaves the manufacturers free to build as much power as they want. The DEF is stored in a separate, insulated, heated tank and typically has a blue filler cap.

There are many misconceptions about DEF, so we thought we'd clear some of them up. For more information, you can visit

1. What is DEF made from?
DEF is a mixture of (typically) 2/3 deionized water and 1/3 urea. It's carefully regulated by the American Petroleum Institute. Technically, urea is derived from one of the byproducts of urine. But it's synthetically made, so no cats are ever harmed in the production of the fluid.

2. Do I have to fill the DEF tank with each fill-up?
No, depending on how much you're hauling, the rate of DEF use is about 2.5 gallons for 800 or so miles of travel. Depending on what year and model truck you have, you could have a fuel-gauge-like readout (Ram), a digital indicator (GM) or a simple "low" light (Ford).

3. You can't get DEF fluid anywhere.
As you might expect, truck stops are likely to carry several brands of DEF, sometimes in multigallon containers or availble at the pump. Here are just a few places we've found that carry DEF: Walmart, TravelCenters of America, Flying J Truck Stops, Love's Travel Stops, Petro Stopping Centers, and Pilot Travel Centers. Plus, you can purchase it by the gallon at many auto parts stores like O'Reilly's and Advance Auto.

4. There are more cons than pros with a DEF-equipped truck.
The only cons we can find are that the trucks cost a bit more, DEF requires some room in your truck, and it adds a nominal amount of weight. The pros include more optimized combustion, better fuel efficiency, increased power, reduced maintenance, fewer regenerations, less wear on the engine, plus it yields harmless nitrogen and water into the atmosphere, and it's highly reliable.

5. NOx isn't a big deal in the first place.
NOx is an element in exhaust that has been blamed for acid rain, smog and raising the overall greenhouse gas levels of the planet. DEF, as part of a SCR system, turns NOx into harmless nitrogen gas and water (which is present in the air we breathe). Whether you believe these man-made chemicals have a role to play in climate change or not, NOx is something we don't need to spew into the environment.

6. On a hot day DEF will evaporate.

At a constant 120 degrees it would take two years for the DEF to turn into ammonia and evaporate. You can stop worrying about evaporation of DEF.

7. DEF will kill my fuel mileage.
Quite the contrary. Because the manufacturer is able to tune the engine any way it wants to, and then allow the SCR and DEF to take care of the pollutants, most manufacturers have discovered better fuel mileage (compared to other smog-reduction systems). Truckmakers claim it improves fuel mileage by as much as 5 percent.

8. This is new technology that has never been used before.
Incorrect. SCR and DEF have been used for decades in other commercial and agricultural applications. We should note, automotive-grade urea has a much higher purity than fertilizer urea. If you use a lower-quality fertilizer urea you risk degradation of the SCR system, which could cause your truck to break down. It may even prompt the sensors to believe that the truck's DEF tank is empty.

9. DEF is nasty and toxic.
Hardly. Urea is the active ingredient in DEF, and it is used in fertilizers, plastics, animal feed, pharmaceutical applications and some cleaning agents (maybe that's why folks think it is toxic). DEF is less toxic than many other fluids in a truck, including diesel fuel, engine oil, brake fluid, antifreeze and windshield washing fluid.

10. What happens to my truck if I run out of DEF?
The EPA requires all truckmakers to incorporate some type of staged warning system (some offer actual gauges) to let the driver know exactly how close to empty the system is. Whether a vehicle goes into a "limp home" or reduced engine power or limits the number of times you can turn the engine on will depend on the specific car or truck, but at some point it will not start. Simply put, you should treat your DEF tank the same way you treat your fuel tank; you don't want to leave yourself stranded by ignoring the warnings.

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Ram Pickup DEF Gauge photo by Mark Williams



Sandman. Firstly what are others "saying about me"? Secondly the entire point, which I have spelled out twice, is that it is difficult and expensive (and not even always possible) to deactivate cylinders with overhad cam engines - and that ALL diesels in the United States will use overhead cam valvetrains. The engines are either I6's, which gain nothing from pushrods and so will use overhead cams, or they are small V engines from Europe, ALL of which use overhead cams because there is a tax advantage to use OHC in Europe.

If you shut down half of your cylinders in your diesel, and boost and fuel injection were increased dramatically in the other half of cylinders, those cylinders would be fried in short order. If you cut half your cylinders and occasionally for a fraction of a second the above happened, over time you would fry those pistons. If boost is increased dramatically and fuel injection is not increased then may still have an issue if you've plugged too much air into the cylinder. Diesels run lean but they can only compress so much air at the end of the day. If the entire system were thoroughly tested and there were no real issues with overboosting upon cylinder shut down or excess fuel injection then it could work, but once again nobody would implement it for the two reasons I have stated twice above:

1.) Cylinder deactivation saves fuel primarily via reducing throttling losses. Diesels have no throttle so there is nothing to be saved here.

2.) The diesels we do have are all OHC, implementing cylinder deactivation is expensive and difficult on OHC engines, and would be worthless for the nearly zero gain such a system would offer on diesels.

mileage man" I will now have to side with Ryan and others here, you are not either able to read, or of a one track mind, as I am once again talking ONLY about ohv engines, like the Duramax, and Fords new diesel engines!!! which are NOT ohc BUT ohv! get it? yet? please do not prove to all of us here you are not of sound mind! as so far you have been doing just that, I have also said I agree with you as far a ohc engines go, BUT once again I am NOT talking about them!!!

to answer your question about over boost to the rest of the cylinders? have you ever heard of a blow off valve? which most turbocharged engines have! all that can be accommodated! with sound engineering!

Sandman you asked why nobody was using cylinder deactivation on diesels and I told you why. Several times. You then started pretending as if you were only talking about OHV engines, which you were not initially - which is fine - but valvetrain actuation ABSOLUTELY plays a role in the answer to your question. If your specific question was "why do GM and Ford not use cylinder deactivation in the 6.4L powerstroke and LML duramax then you should have asked that specifically - also the answer was provided for you several times. It won't do anything. You cannot save fuel reducing throttling losses when you have no throttle to begin with.

Very few diesel engines have blow off valves, and it would not be a trivial task to set one up properly with cylinder deactivation. You would need an added solenoid to cut off the inlet and open the outlet. It may need to have a variable amount of actuation (multiple solenoids). And of course, the MAIN PROBLEM, which I have been stating ALL ALONG is that you would have to be very good with your control software to avoid surge issues.

I don't know who "Ryan" is, and I have no idea what he has posted about me, nor do I care. What I can tell from this discussion is that you are an idiot. You accuse me of not being able to read when I have posted the reply to your (very simple) question three times now. YOU need to read. Or maybe just go back to school before posting here again.

mileageman Now we all know for a fact you can not read, and before you get all up and defensive, go back to my first post, and I did indeed mention a Dura Max! and there is also a sentence there that says, "after all we are only taking about pushrod engines anyway!"

Sandman I am not defensive, I am annoyed that you are a blithering idiot incapable of reading the same response given to you three times that answered your question. That you would then go on and discuss reading comprehension after literally failing to read the answer to your question three times is essentially proof positive that you are nothing more than a troll.

just a name calling liberal troll that has no idea what I am posting that is all, yes.

Just to be clear, NOx as a regulated emission is not considered a GHG. NOx is typically considered to be NO + NO2. N2O is a GHG, but is typically not considered NOx.

NOx is regulated because it contributes to ground-level ozone production (ozone is the primary constituent of "smog"). However, in some local ambient air chemistry conditions (known as "VOC limited"), NOx actually destroys ozone. VOC, primarily from gasoline exhaust and evaporation of gasoline, is the main culprit in ozone formation.

A good article on this phenomenon is available at

@LCM, and you'll agree that the concentration of these specific gases makes all the difference.

If you live in Mexico City or LA the oxides of nitrogen are a significant issue on some days. In Tahiti, probably not so much.

@papa, don't know about Mexico City, but LA is "VOC-limited" with respect to ozone formation. Thus, decreasing ambient NOx concentrations without a corresponding decrease in ambient VOC concentrations will likely result in increased ambient ozone (i.e., "smog") concentrations.

This is fundamental atmospheric chemistry and is something even EPA has acknowledged.

Direct quote from an EPA document ("Final Regulatory Impact Analysis: Control of Emissions from Nonroad Diesel Engines.")...

"...While this final rule will reduce ozone levels generally and provide significant ozone-related health benefits, this is not always the case at the local level. Due to the complex photochemistry of ozone production, NOx emissions lead to both the formation and destruction of ozone, depending on the relative quantities of NOx, VOC, and ozone catalysts such as the OH and HO2 radicals. In areas dominated by fresh emissions of NOx, ozone catalysts are removed via the production of nitric acid, which slows the ozone formation rate. Because NOx is generally depleted more rapidly than VOC, this effect is usually short-lived and the emitted NOx can lead to ozone formation later and further downwind. The terms "NOx disbenefits" or "ozone disbenefits" refer to the ozone increases that can result from NOx emission reductions in these localized areas....

...EPA's air quality modeling predicts NOx disbenefits in the areas identified by some studies as "VOC-limited" (e.g., Los Angeles)...." (Page 2-113)

I really enjoy all the talk about eliminating the EPA, and how the regulations are not needed well let us not forget who it was that passed all these pollution regulations in 2004, that would be a Republican president, senate, and house.
The climate change deniers are funded by the same advertising agency that did the smoking is good for you campaign.
Pollution to you may not be a big thing, but if you child has azma then it might mean a little more.
I have a NBS 2007 GMC the thing is great and it has a particulate filter, if you can not afford the .25 milage increase, because of regenerating then you can not afford to drive it period.
Just a thought!

will this system clog my exhaust system after 50,000 plus miles? and if it does what do I do?

Why doesn't Bob Carpenter just jog on over to the nearest Toyota dealership and get himself a new Prius. Because that's what he's looking for in a truck. I'm sorry, diesel trucks are not going to be clean. They were meant to be dirty and loud. That's what makes a diesel a diesel. He says at point #7 "Truck makers have claimed an increase in fuel mileage by as much as 5%". First you cannot trust truck makers. They will say anything to get you to buy their product. Secondly, just adding an aftermarket, turbo back exhaust increases fuel mileage by about 7%-10%. So you're telling me that a highly expensive and complex system yields less increase than a simple "cheap" (in relative terms) system. Yeah its way more efficient. Programmers have been able to achieve 10%-15% in increase. You add up all the fuel mileage increasing products that don't limit emissions and you've got about 20%-30+% increase. Obama and the stupid EPA are killing whats great in this country.

What a load of crap. I took this system out of my truck and got another 400kms out of my tank. Fuel milage savings? Gotta love how marketing controls the world. Open up your eyes people!

I lived in California for more than a decade (mostly before the diesel emissions kicked in) and I never really saw this pollution thing they keep talking about. The only thing I saw was that the same truck cost $50k before and costs $70k now.

I guess the one environmental benefit we might get is that folks can't afford a new truck and will drive their old one longer.

To my amusement a couple of months after these urine trucks rolled off the showroom I saw complete systems for sale in the classifieds, removed from the trucks..

A lot of bad information here as shown in other posts. I have a 2008 Sierra HD Diesel and any diesel truck under 10.000 pounds does not require any inspection here in New Jersey.

I have a 2012 F350 with the 6.7 Powerstroke, had for about two and half years now. Today I had my truck towed to the dealership for the 5th time since I bought it due to DEF systems issues once again like the previous 4 times. The DEF system is a waste and does nothing but cause more issues and cost more money for diesel owners. Gas mileage is not improved by the DEF, at best I get 12 mpg. Once we took my buddies DEF system off with no tuning and his mpg instantly shot up to 16 mpg plus. Then you have all the sensor issues, which they never conveniently go out at the same time and when they do decide to go out they shut your truck down completely no matter where you are. Which also conveniently is one of the things FORD does not cover on their 200,000 extended warranty. They know these DEF systems will fail and have many issues to be worked out so they dont warranty any of the DEF system of its parts. This was a horrible article with little research or one sided research done. Come out to the West Texas oilfield where a large majority of the population drives diesel trucks for our jobs. Come out here and see just how bad the DEF has affected the diesel market out here. I could go on and on about the two week old 1,000 mile GMC diesel my dad just bought that is in the shop for DEF system issues.

Wanted to know if DEF fluid freezes?

Of course the person that wrote this is being paid by api,epa, trucks manufacturers etc,, what this article don't say is that DEF was promised by everyone including EPA at 25 to 70 cents per gallon, that was the big scheme/hoax, while i would care for environment i must paid at least 14 dollars per 2.5 gallons,do your arithmetic and see, let alone the horrible quality of def at the pump in trucks stops stations, more break down, more money to spend at mechanic shops.

Did my own testing


@Dave you obviously don't know anything about motorcycles the pipes are basically giant mufflers unless you take the baffles out there is a real point to motorcycles being loud it's so that people notice them. most people don't notice motorcycles because they are looking for cars and trucks when people start seeing motorcycles every time then i will put my baffles back in until then my bike will be loud and proud for my safety

The technician put this stuff in my windshield washer fluid reservoir. How do I get it out? Will it damage my paint as it sprayed all over? Will I need new lines put in to clean it all out?
When I sprayed it on my windshield last night at dusk, it completely oblieterated my view with a "waxy" film. It took elbow grease and lots of winded just to get it off my glass

Is anybody having trouble with
contaminated DEF. This has been
A continuing problem with my
2011 DuramX. According to
My Dealet anytime the bad
DEF warning comes on, they say
It is due to the DEF. shelf life.

My 1500 eco diesel is back at dealer for the second time. I am going to try and find the best DEF out there and return my peak def to walmart. I guess texas and florida weather doesnt agree with def lol. Guess someone should tell mother nature that it has to cool down so our trucks can run right. funny my 06 dodge cummins and vw Tdi will never have this problem. I met a woman while dropping off my eco diesel she has had her truck in 5 times since february. Should I buy mopar def not sure what to do anymore.

Can I fake out the DEF system by just filling the tank with water?

How about water and a little pee mixed in?

Water is a bit cheaper than DEF especially when all the stores are closed and you need to get home.

When you disengage the system and bypass the warnings your fuel economy goes way up

Let the EPA pay for the DEF if it's such a great thing
What should diesel owner be singled out
Or is I by a Gas truck I do not have the cost of this,
I have had many truck owner fuel system crystallize and have to spend five thousand on a new fuel system! Ford make a kit for this problem
So buy gas and burn the hell out of it why try to save fuel?
Or put the EPA in the unemployment line.that would save fuel too

Thank you for the info. Great article

Do you have any information on DEF becoming contaminated? Possibly too old, cold or hot that would cause crystallization? How do you test DEF to see if it is good?

number 4. Pros and cons. I can tell there is a big con with DEF systems. It will cost you $1800.00 to replace the pump when it goes. Mine went at 190000 KM. If the system was not in the truck I would not have that bill.

Does the DEF @ ROTTEN ROBBIE have an ISOLATED # .

You can't really believe that def is a good thing. Yes it's injected into the exhaust, not the motor, so how does that increase power??

Then there is the fact that it causes a lot more issues then normal, it freezes, mechanical issues, causes electrical problems, and on and on. Plus you can't deny the fact that it's just another cost to run your vehicle.

Def, ok so it needs to be processed shipped, boxed. Ya def saving the environment one plastic container and box at a time.

Now the facts, I have worked and talked with tons of people and mechanics about the def and egr systems in vehicles today and we all come back to these facts. We have monitored vehicles with these items installed and we have monitored vehicles with these removed.

Facts. Def is an added cost to the consumer, another thing to fix/ deal with. Def cost money to produce, box, ship, causing more waste and pollution. Most vehicle def tanks are designed to hold enough def fluid to make it to your next oil change, some vehicles don't even make it 1/10 of the way to there oil change and need to be refilled. Vehicles with these systems removed gain power and a minimum of a 25% increase in fuel milage, most a 50% increase in fuel milage. Some as high as a 75% increase, not kidding.

OK so, def makes the gov and producers cash. Plus all these environmental companies... aka more tax revenue. You cannot tell me that the simple math into fuel economy is wrong. Def is suppose to help with exhaust pollution, but a system that consumes twice the amount of fuel... common your all just retarded to think that this is better for the environment. The added materials incorporated into the exhaust system like nickel, the pollution created to harvest these material, exceeds alone what we are suppose to be saving. Oh and then to dispose of some of these materials, oh wait that's if you actually can, cause some of them there is no recycling these materials. Toxic, no renewable waste.

Common, seriously there are so many cons to these systems that I could list on and on and on. The only pro's to this stuff, def brakes down some of the chemicals released into the enviroment/ air, and it stops the build up in some areas. How ever it then singles that build up in one area. The cost of all this and the simple fact that we consume twice the amount of fuel with these systems in place, don't tell me it's good for the environment. It's good for the pocket book is all, just def not are pocket books.

I believe that "man-made climate change" is pure mythology. But SMOG is another matter. Nitrogen oxides are horrible. Smog kills people.

I live in northern Utah. A long way from LA. We have very serious problems with smog. It's not theory. You can see it, smell it and taste it.

About 10 years ago I visited Russia, and driving into Moscow the first thing I noticed was that the air was physically painful to breathe. Throat burning, coughing, wheezing, and the smell was acrid and horrible. But it was strangely familiar too. Then it hit me: My early childhood was in the pre-emission control era, and that's what HOME smelled (and felt, and tasted) like. Far worse than today, even though back then the population was about 1/4 what it is today.

You won't find anybody more conservative than I am, but that doesn't mean I want to wreck my lungs. I'd take up smoking if I wanted to do that. In the meantime urea injection in diesels works well, at a relatively minor expense. Does NOT hurt performance. (Allows it to be better, actually.) So why not?

Cost of repair and maintaince. Most people posting here are not well informed. The cost of a new catalyst for a 2010 Kenworth is 8000.00 with installation you are looking at 12000. It use to be fuel and tires that were the highest cost annually for highway tractors. This is no longer case. The highest cost now are all the sensors valves and other engine emission equipment on the trucks. That does not include the cost of the def itself. A egr valve for a volvo truck is 1000.00 there are 2 of them on the trucks. That does not include the cost of installation. Add to that that most repair shops are clueless and to diagnosis by guess and replace till they hit the right part that was the problem. It is becoming common to find trucks that have the system deleted and the parts sitting there just for show.

My 2015 GMC duranax bit the dust at 59,000 miles because of DEF system, yep bankrupted me, I lost my job, pickup, and life over it. Maybe the real question is who profits from the real cost of DEF systems. I personally believe it’s a financial weapon to destroy.

Hi Bob, On item 4. on DEF you have an
error... You state there are more cons than pros. If you read number 4. you state more PROS not CONS. No Biggie. Thanks for your article.

Road Trip, having a EGR raises the nitrates in the down stream exhaust, DEF injected down stream helps to strip out the nitrates turning them in to just water and nitrogen, so the EGR is used on all diesel engins that use DEF to treat the exhaust, the bad side is it is exhausted out on the road ways which turn in to water ways every time it rains increasing algae blooms in the wetlands






We have two tyco yard truck that use def .These truck only run ten mph and the regin does not work and these truck stay broke down all the time.they are 2017 truck we have to get a company to come out and put a computer on it. then it will run for little while and quit again

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