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



I have it on mine. It is not a big burden. I fill it when I change the oil at around 5K miles.

@Bob Carpenter

Another discussion of tack-hammers and sledgehammers. For people who live in Mexico City or LA or Beijing, oxides of nitrogen and hydro carbon emissions have an effect on quality of life. While these are very populous cities, their total numbers make up a teeny fraction of the whole world's population.

People who live in Tahiti, or Bermuda (or Idaho?) have different needs and this kind of approach to air pollution ends up being a sledgehammer--when NO HAMMER was required.

There is zero scientific evidence that can trace the emissions of a vehicle in Bermuda to any harm experienced by someone in (name your city). It is a broad brush approach to a problem experienced by a few of the world's biggest cities.

#5 Should read "Whether you believe these man-made-up storries about climate change or not, NOx is something we dont need spew into the air"

Climate change Scientists recently admitted they falsified climate chang data to fit their agenda, lets not give them credit for the hoax.

I know this is a little off topic but all of you guys knocking chrysler because of fiat need to realize something. the world is a global market. gm has owned a percentage of fiat at one time. gm still has many other global brands. ford has owned other brands. where do you think all of these heavy but so called safe unibody fwd ecoboost cars came from? volvo. and chrysler is still a u.s. entity. and a little history; ford's tractor company was originally called fordson. fordson quit making tractors in america. then, ford brought back tractors to america but just started calling them ford tractors. then they wanted to get out of tractors completely and bought new holland and merged ford tractors with new holland. they made a deal that new holland had to still use the ford name on tractors up unto the 1990s even though they were new hollands. meanwhile, fiat who used to make tractors bought out case and pretty much did the same thing that ford did with new holland. then fiat bought out new holland from ford and merged it with case to form case new holland. and the ford/new holland deal about fords name still carried over so some case new hollands under fiat had fords name on them. and guess what? now fiat has chrysler. and they split ram from dodge. and they showed a yellow ram concept with case on it. and a case backhoe had a ram laramie longhorn interior. so fiat might merge ram with case new holland. after all, its all agricultural/industrial. and it might be a good thing. and it would be good if ram has its roots in agriculture, just like the superbowl ads. and all of you guys picking on a global company that has the potential to go far. i believe that if case new holland is merged with ram, that is what will get them to outsell gm, and eventually in the long term, ford. agricultural and industrial vehicles give people a percieved quality. but my point is the market is global, and the possibilities in this world are endless, so the fiat crap is ridiculous. just because fiat is getting some money off of a ram doesnt mean that ram still isnt making the vehicle for the ones that buy it. and either way they are still making them in michigan or saltillo, which saltillo has a higher quality rating anyway. and its still north america. my god people.

This past winter was so cold (here in Toronto) that quite a few of our trucks had the def freeze.

On #2, 800 miles seems like an awfully short refill cycle. Did you mean to say 8,000?

No kidding. The freeze point is just -11 deg C.
To unfreeze that costs energy and Mpg every morning. No, thank you . I will stick with my plan to buy RAM 2500 with 6.4 HEMI.

Some more useful info.

#4 is BS. DEF is sprayed into the exhaust. That does not affect combustion or fuel efficiency. Those improvements could be made without DEF.

"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."

Bi-annual inspections? Really? Not that I know of. I had a 2007 Dodge Ram 3500 with Cummins diesel from early 2007 until early 2014. Never was there any requirement for an inspection. Maybe in California but not where I was from at the time. Perhaps the trucks using DEF require that but it sure wasn't a requirement for mine. Mine had the diesel particulate filter but not DEF.

Way to have an article called "Top 10 Facts" that doesn't have all facts and a lot of opinion

For one thing - how do we know that the production to create DEF and the costs of manufacturing all the DEF components doesn't harm the environment more then having the increased NOx? Is there a study on that?

I know of no "bi-annual" inspection in my area as well

Now we need the EPA to require catalytic converters and mufflers on motorcycles. no pun intended. they should.

A generally bad piece, this website would be better served putting out fewer articles than by putting out garbage written by Bob Carpenter. NOx is not a big deal in almost all locations around the globe, SCR is just a large added expense and power robber while offering nearly zero positive impact for the environment. SCR systems unequivocally hurt fuel mileage. They add weight, they add complexity, and they certainly do not allow engine makers to "tune the system the way they want to". Get real. SCR systems allow for improved mileage in DPF outfitted vehicles, because they do help to reduce regens in most DPF systems. But that is not what was written, likely because the author didn't understand it. Water/methanol injection is a much better way to reduce dpf regens and add power to the truck.

@Don E.
#4, yes they could be made without DEF, but they would not comply with US emissions regulations, and therefore could not be sold.

Dave, We don't need the EPA to do anything. We need to abolish the EPA and everyone will be better off.

Abolish the EPA statement brings about a few questions?

Why do you want no emissions requirements?
Do you want want higher level emissions than the EPA allows?

Yet more uninformed idiots who think they're smarter than the rest of the world commenting here......probably the same ones still claiming cigarettes don't cause lung cancer.


I was just going to say something similar. Thanks Bob for the article.


The only "uninformed idiot" who thinks that he is "smarter than the rest of the world" that I have seen post here so far is you. Feel free to stop posting.

Alright now, boys!

Do I have to send you to your rooms?

Seriously, cigarettes? Smoking causes cancer???

Living causes cancer. I'm an expert.

You cannot get cancer if you are dead. I've met guys in their 80s who smoked like fiends all their life and did not get cancer.

My uncle died of lung cancer and never smoked. He survived being in a Nazi prison during WW2, but died of lung cancer.

Regarding cancer, smoking and pollution.

In a perfect world we don't smoke and we don't pollute. However, there are better things for our nation's leaders to be all revved up about than Keystone XL or tobacco.

I do not elect Senators and Presidents to be my nanny. I can make my own decisions, whether we're talking about tobacco, diesel, (or strong drink).

Well, one thing we can all agree on is the ridiculous placement of the DEF tank on the GM I right?

The reason trucks using DEF get better fuel economy is simple: DEF reduces the need for efficiency-robbing EGR. When the engine has to regularly reintroduce dirty air into the intake system, fuel economy goes down. When aftertreatment can handle the same job, the engine uses more of the efficient clean air. A separate tank of fuel for DPF regenerative cycles would improve fuel economy even more, since the main diesel fuel tank would then only be used for engine combustion.


Promise that I'm not being a wise ass, here. My experience with EGR is limited to gasoline engines but I think the principle is the same in each case: Introduce some exhaust into the cylinder under certain conditions to reduce the temperature of the combustion.

Benefits to include longer wear of key internals, reduced NO emissions and reduced detonation.

Are these the same with diesel?

I had four different cars/trucks with gas EGR that never caused me any problems and one that was a complete stinker. The EGR in my 1994 3.8 mercury was a problem that cost me plenty and the engine died with less than 100k on the clock.

The rest gave me great and long life.

@papa jim
The last thing in a diesel is the want to reduce detonation.

@ papa jim - with all due respect............cancer is caused by a mutation of one's own genes and one's own checks and balances either miss the mutation or the cancer cell "protects" itself.

Some people are more prone genetically to develop certain cancers. Some just need a trigger to start the process and pollutants can be that trigger and can also be the direct cause.

I'd rather take the side of caution and reduce/eliminate potential carcinogenic substances.

papa jim, since diesels don't ignite with a spark, they need a good air mixture to achieve combustion. Increasing "dirty" EGR air reduces the efficiency of the combustion cycle because it won't compress like clean air.

Chrysler and Cummins were able to achieve 2010 standards in 2007 with precious-metal catalysts in place of SCR/DEF, but required increased use of EGR to do it, and mpg suffered noticeably. Those trucks are the least sought by used Ram buyers for that reason.

@papa jim and Dave,

Why do you want dirty air and dirty water?

This may not be part of the exact subject, but I have a question for the engineers out there, and that is if DOD, or AFM, or any of the other gas saving ideas in gasoline engines used today can save decent amount of gas in gasoline engines, why has this NOT been talked about, and used in diesel engines? I mean if it can be done is a gas fired internal combustion engine with direct fuel injection, with the use of electronic solenoids used to control the certain valves from opening in a timely manner in say half the cylinders, then why not is a diesel engine that has 400hp and 800+tf/lbs tq, when you are not using them? all? can anyone imagine the mpg these diesels engines would get, when just loping down the hyw unloaded? I all the years I have been reading about these things, there has never been any mention of this? and we are after all talking about OHV pushrod engines anyway, that just happen to use diesel instead of gasoline? anyway! Just imagine a Duramax, with say a higher state of tune, with say 450hp and 85ft/lbs tq, using GM's AFM, and while loping down the hyw unloaded getting over 20mpg? if it works so well in the 6.2 EcoTec 3 engine, with over 400 hp and 460ft/lbs tq, and that engine gets over 20 mpg, why not a diesel?

@sandman4X4 - there is new evidence from Europe that higher compression DI/turbo engines have similar emission patterns as diesel engines. fine particulate is often higher.

If those finding prove correct and gain traction within EPA "type" agencies we may very well see DEF systems added to gasoline powered vehicles.

If DEF "makes life easier" for engine designers, it may become viewed as a necessary evil where one can have their cake and eat it too (higher HP/torque with better emissions).


"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."

Is chemistry your specialty or journalism? Just asking.


Do you have a link for the new evidence in Europe statement? That is an interesting point and could be a big shift in future truck development. I'd like to read more about it.


NOx is nasty stuff.
I am a firm disbeliever in the global warming myth, it is nothing more than a $$$ generator for the enviro-nuts.
That being said, anything that reduces or eliminates stuff like NOx is a good thing. The auto industry has adapted to it and I think it's a good thing.

bring back the 7.3 and crap put the emissions backfire.

To add to number 3 on the list. Another good place for people to get DEF is heavy duty truck dealerships. Due to the amount of of DEF class 8 trucks use, we keep DEF in large quantities and the pricing would be better than most trucks stops or other light vehicle parts stores due to how competitive the heavy duty parts market is. People think that light vehicle auto parts stores are inexpensive but they make big margins on stuff like that. For example, a pack of 5 butt connectors would cost you $5.00 at an Auto zone type store, but are about $.20 each at most of your heavy duty truck parts dealerships. You can also get Cummins parts cheaper at these places too since the ISB 6.7L and old 5.9L is also used in some medium Duty class 6 & 7 trucks.

@Tim Esterdahl - it was a story on TTAC a while ago. I'll see if I can find the link.


Every area of public policy (and science) has trade-offs.

Agree that in concentration oxides of nitrogen can be dangerous.

So instead of building EVERY car or truck on earth to address the needs of a few relatively small areas, why not focus the limited resources available to the areas that have a problem and let everyone else (the vast majority of the world's population) go on about our business?

Check the world map on Wiki to see the few zones worldwide where NO2 is at concentrations that cause concern.

Let's focus finite resources elsewhere to making sure that people have enough to eat, that nations aren't building huge nuclear arsenals, and that countries ostensibly using nukes to make electricity are not in fact making bomb grade materials.

During the last five years our government has apparently ignored the scariest players on the world scene, but we focus a lot of attention on people who want to build oil pipelines and coal-to-fuel plants.

@Tim Esterdahl - here is some light reading for you ;)


First and foremost the reason to use cylinder deactivation is to reduce throttling losses - which occur when the throttle plate is not fully open and a portion of engine energy has to be wasted pulling air past the plate. Diesels are unthrottled (egr is throttled but that is a different story) so this setup would not help them.

Secondly it is not particularly easy (and certainly not cheap) to deactivate cylinders if you do not have a pushrod engine. DIesels typically come from Europe where the idiots tax based upon displacement, where DOHC setups have an advantage (about the only reason to use DOHC in consumer vehicle "V" engines"), and in line configurations where you may as well run overhead cams since that advantages of pushrods are lost in such a configuration.

Finally all diesels are turbo'd and it would be difficult to setup the turbo and cylinder deactivation to work seamlessly together.

Setup a pushrod V type diesel/electric hybrid that uses exhaust to run a generator instead of a turbo and you might be able to do some impressive things with inactivation. Thing is with a modern diesel you will get much better gains by capping off the EGR, removing the dpf, and possibly adding a water/methanol injection system.

I do think some people just don't know how to use Google!

What will be amazing is if the US government screw the energy supplier of gasoline like diesel is being screwed in the US.

Will the US vehicle manufacturers, EPA, energy, farm lobby, and government brush this one over?

@DeverMike/Paul/Tom Lemon/Greg Baird/TRX4Tom/Dave/Hemi V8/Tom Terrific/sandman 4x4/lautenslager/zveria/Bob/US Truck Driver/Glenn/Jason/Hemi Rampage/smartest truck guy/Maxx/SuperDuty37/Ken/Ron/johnny doe/jim/ALL1/Frank/Idahoe Joe/The Guy/AD/Casey/papa jim/Young Guy/BeeBe/Steve/Chris/The truck guy/Alex/Mr Chow/Yessir/All Americans/Scott/Buy American or say Bye to America or whoever you want to call yourself.

Quit the crap, really.

It's getting long in the tooth.

You want to debate, but it has to be on your terms.

Learn to debate with good information, then we might be able to have a decent debate.

Opinions are good, but if they are only your view to support the UAW, then how good are they. Look at what you guys have done to Detroit.

Terror tactics (union tactics) don't work on me.

If PUTC wants the UAW or whatever to control this site I suppose it's their decision.

It's not kids like I've been told by PUTC.

They don't seem to care. So this will go on.

A lot of these third world countries are recieving pickups that are more technologically advanced than what you have. The recent NA Ranger is behind technologically than the Great Wall pickup from China.

Even your Taco's technology is behind the Hilux.

These new third world pickups have a 5 star safety rating, far more than what is being offered in any NA pickup.

More hp doesn't equate to technology. Even in NA there are alot of people who couldn't afford to run a 400hp vehicle.

The US is in a lot of debt and large contributing factor to your debt is the importation of oil. Use less oil and help the US recover, use diesel.

"The US is in a lot of debt and large contributing factor to your debt is the importation of oil. Use less oil and help the US recover, use diesel."

Why are you (and others?) concerned about US (publicly held) debt? Those who lend should be concerned, but if you are not lending to a person, corporation or government, why not worry about something more important.

Government debt is a boogyman. Ditto for oil consumption. This is the nibbling-around-the-edges kind of stuff in regard to public policy or industry.

The 800 pound gorilla is banking. 150 years ago, banks printed their own gold (or silver) certificates, i.e., paper money, and the mischief and mayhem that resulted was bloody, tragic and unnecessary.

The world today is on the brink of another such era because governments now run the banks and they're printing funny money. We could have a system today with paper money (or electronic credits) that are backed by something of value, but unfortunately, the horse is out of the barn--politicians love the power to "make" money.

Oil consumption and debt, not so much. Sure the US and EU have a lot of debt but there are wealthy persons and institutions worldwide who gladly hold that paper and stand in line to buy more. Funky currency is the problem because it's crushing the middle class.

Ask me why, Al.

My only complaint is the limp mode. I understand that it keeps people honest but the price of DEF can get pretty expensive, I'd rather see a control system put in place like what they use for evaporative emissions if your gas cap doesn't seal then you get a CEL and loose cruise control. I would hate to driving in a remote area like the desert and due to some malfunction have the truck go into limp mode.

Other's on this site have attempted to travel down the path you are heading.

The reality is this, US pickups are great vehicles, but as in this article does allude to they are protected, their permanence isn't very secure.

It's not much different here with our pickups and utes.

They are mainly SUVs and toys, they have be migrating in that direction for many years.

CAFE alone is designed to reduce the number of large vehicles on US roads. Even if CAFE is friendlier to pickups. But, how friendly is CAFE when you need to build a full size out of aluminium and run quite small V6's that are turbo'd. Again increasing cost.

Aluminium and turbo'd gasoline engine starts to make diesel more attractive.

Diesel or gasoline, I really don't care. But my interest has been the general direction that the industry has been heading in. The trending of information and data. Many of you might not like my prognosis, but I try and remain objective.

I'm don't have the emotional attachment you have with Ford and Eco Boosts or Hemi V8 with Rams or any of the other fan boi's.

Gasoline offers advantages as does diesel. Currently diesel is more advantageous to achieve better results than gasoline just due to it's density. Gasoline can't achieve the FE advantage of diesel. Unless by some magical sorcery you are an alchemist.

Gasoline and diesel can both become more efficient.

Diesel regulations in the US are actually trade barriers like this chicken tax.

The US uses a different quality of diesel. It is more abrasive, contains more sulphur and has a lower cetane value.

This doesn't allow for the same engine that the Europeans can run. But, attaining the emissions standards in the Eurozone is becoming harder as well. US diesel doesn't allow for low enough compression of diesel engines to maximise the reduction of NOx. NOx is a big hurdle with diesel.

Gasoline might head down that track with particulates, we'll see.

Don't worry about an FTA between the US and the Japanese as Obama failed and I bet vehicle tariffs (chicken tax) and mainly agricultural protection are to blame.

You only have to worry about the US reaching an FTA with the Korean's. But the Korean's are similar to the Japanese with protectionism.

This is a pity as the Japanese and Korean's are two very significant players in the Pacific region.

The US and Asian's have to be able to play together a little better.

This show that the US's attempts in the East Asia region to increase it influence isn't as great as most would have liked.

@Big Al

Instead of worrying about oil or gas, considering getting a grip on caffeine. Did you read you last comment before pushing the button?

Aussie Aussie Aussie

Oii Oii Oii

mileageman: I suggest you look into just exactly what I am posting, and I said nothing about European engines, but I do mention nothing BUT pushrod engines! and what the system does do is use an electric solenoid to close a certain valve, and in turn that lets the cylinder run without any fuel or air and no vacume, so even an engine with forced induction the only thing that can happen is to save some fuel, that would not be going into the closed cylinder. nuf said!

Sandman what are you talking about? I replied directly to your question. Cylinder deactivation is used to primarily reduce throttling losses, which diesels do not have. It is also difficult and expensive to use cylinder deactivation with OHC valvetrains. The diesels in the US are either from Europe (which will always be OHC due to the stupid tax policies there) or larger inline configurations which get no advantage from using pushrods and so will not use that valvetrain setup.

With forced induction you will have to have your timing just right, or you will not have a pretty engine compartment. 15 pounds of boost to 8 cylinders will result in 30 lbs of boost to 4 cylinders if intake valves are shuttered. If you try to inject fuel to be stoichiometric you will blow up those cylinders, if you do not add fuel you will run extremely lean, will not pass emissions, and will probably end up melting the piston. Now it is unlikely that a high boost situation would ever be met with a cylinder deactivation situation, but nevertheless over thousands of miles if there were any broken overlap there you could be causing quite a bit of grief with a system like that - it would have to be very thoroughly tested.

As an owner of a newer diesel and have owned an older diesel, I thought I would hate having the DEF and smog equipment on my truck. The debate can go on and on about global warming and what not. The fuel mileage between my 05' Duramax and 11' Duramax is that the 11' gets better overall mileage. The best part is when idling and getting my camper on my truck I don't have to smell the exhaust.

mileage man: once again I AM NOT TALKING ABOUT OR HAVE NEVER POSTED ABOUT OVER HEAD CAN ENIGINES!! and I do agree with you on that, and that is why I do not mention them, but as far as the engine I do mention? they have NO over head cams! Please do not still respond or I will start to believe in what others have posted about you! As far as a diesel melting a piston? when the cylinder it is in is shut off? I would say that is not possible! ONCE again I am NOT talking about OHC engines! Or anything made in Europe! Now maybe some engineer can give me a real answer? on why the Duramax can not have AFM?

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