33 Answers To Your Diesel Exhaust Fluid DEF Questions

33 Answers To Your Diesel Exhaust Fluid DEF Questions

I’ve seen a ton of questions out there about Diesel Exhaust Fluid, or DEF for short. I have been a professional diesel training instructor for six years, so let me explain what it is and then answer your questions!

DEF Overview

It is a fact nowadays that diesel emission legislation is becoming stricter and stricter. One of the pollutants that interest the legislators particularly, is Nitrogen Oxides (NOx). With every new legislation, the maximum acceptable levels of pollutants change.

For example, when we compare the change of the Nitrogen Oxides (NOx) limits between EPA2007 and EPA2013, it went from 1.2g/hp-hr to 0.2 g/hp-hr. That’s a massive decrease of 83%.

To most of us, numbers mean nothing at all. From an engine manufacturer’s perspective however, this means money. Money spent on research to develop a new engine, or modify an existing engine in order to meet the new standards. Both options can cost from a few millions to maybe billions of dollars.

With the current economic climate, companies try to be as efficient as possible on spending their research budget. The fight to reduce NOx without the need to redesign the whole engine has intensified. Since around 2002, SCR (Selective Catalytic Reduction) systems that use Diesel Exhaust Fluid (DEF) made their appearance in the automotive market.

So what’s in DEF?

DEF is a solution (or mixture) of 32.5% Urea and 67.5% de-ionised water. The solution is injected in the stream of hot exhaust gases and is carried to the SCR catalyst, where it chemically reacts with its surface material and NOx.

The result is remarkable; DEF breaks down NOx to H2O (water), CO2(Carbon Dioxide) and N2 (Nitrogen).

I know a lot of you will read the above paragraphs and wonder how a Urea-based liquid, is injected  at the exhaust system and give water? Keep reading and it will all make sense in a bit.

Diesel Exhaust Fluid (DEF) Questions & Answers

DEF road sign

So on to the questions that you guys and gals have been asking. If you have a question not answered below, please leave a comment below and ask your question, I’lll be sure to update this article!

DEF definition and properties

What is in diesel exhaust fluid?
Diesel Exhaust Fluid or DEF for short, is a mixture of 32.5% Urea (for the ones that like chemistry: (NH2)2CO) and 67.5% de-ionised water.

How does diesel exhaust fluid work?
It is injected into the exhaust stream, where it’s water evaporates. The separation of Urea and Water due to heat, is called thermolysis. The chemical reaction is:

(NH2)2CO + Heat → NH3 + HNCO

NH3 is Ammonia and is the substance that reacts in the SCR and not Urea. HNCO is called isocyanic acid and cannot be used in this form in the SCR.

Isocyanic acid is converted further with the help of the water that got evaporated from before. The process is called Hydrolysis:

HNCO + H2O  → NH3 + CO2

As you can see there is more Ammonia created for use, but as you can see also, the process releases some CO2. By using Ammonia, the SCR system can target and reduce NOx.

Is DEF made from urine?
Even though the name suggests so, Urea that we use nowadays is synthetic


What happens when you put diesel in the DEF tank?
The seals and  that exist in the DEF system are designed to withstand exposure to Urea. If someone makes the mistake and puts diesel fuel in the DEF tank, there is a great risk of the seals becoming contaminated. This could also cause premature deterioration and subsequent failure of one or more components that are designed to handle DEF. If diesel has not been circulated in the DEF system, there is a great possibility that damage can be avoided just by flushing the tank.

What happens if you run out of diesel exhaust fluid?
Diesel exhaust fluid is used to reduce NOx emissions. You will get a lot of warnings to refill the DEF tank before you run out. If you decide to ignore the warnings or the circumstances do not allow tank refill, the engine is designed to limit its power and fuelling dramatically. This is a form of punishment to the driver if you like, but also it is a way of mitigating and controlling the increase of NOx emissions due to the lack of fluid.

Is it bad if I ingested DEF?
Drinking it should be avoided at all costs. If that happens, contact a doctor immediately. Also, refer to any warnings that may be present on the jug or package (if applicable).

What happens if DEF goes into my eyes?
If it gets into your eyes, flush with large amounts of water immediately for at least 15 minutes. Consult a doctor if you have any doubts or  symptoms that are concerning.

DEF Fluid Jugs

What happens if I spill DEF, how do I clean it up?
DEF residue can be easily cleaned up by washing the area with warm water.

Is DEF fluid corrosive?
It is corrosive to most metals used in automotive applications. Carbon steels, iron, zinc, nickel, copper, aluminum and magnesium are some of them. If it comes into contact with any metals, I suggest to clean the area with plenty of warm water to elimitate accelerated corrosion.

DEF Handling / Storage Questions

Can DEF go bad?
The fluid typically has a shelf life of 18 months. It does not expire technically, but it can lose its water content and the ratio of Urea/Water could change. This will affect engine emissions, since all the calculations in the ECU expect urea to be 32.5% of the DEF solution injected.

How can I check the DEF quality?
It’s quality can be easily checked using a urea refractometer (Amazon link).

What happens if DEF freezes? How can you keep DEF from freezing?
67.5% of DEF is water. DEF freezes at -11oC or 12.2oF. When DEF thaws and goes back to liquid form, it maintains it consistency. It is suggested that you store DEF at a temperature between 5°C and 25°C (23°F and 77°F).

How do I store DEF?
Store in sealed containers to avoid contamination and avoid direct sunlight to prevent water evaporation.

How long can you store DEF? Is there a shelf life?
It can be stored for 18 months at a temperature between 5°C and 25°C (23°F and 77°F). For every 5° (9°F) above the recommended temperature, shelf life is reduced by six months.

Other Questions

Is DEF fluid all the same?
All DEF fluids that are supposed to be used in automotive applications should meet ISO 222411-1 specifications. Assuming that all Diesel Exhaust Fluids meet the ISO specifications, they should all be the same.

How much DEF does a diesel use / consume?
It depends on many factors. Some of them include engine size, driver, DEF system condition, presence of NOx in the exhaust stream. Typically, manufacturers refer to DEF consumption as a percentage or fuel consumption. This can vary from as low as 2% to as high as 10%. So if your engine consumes 100 gallons of fuel, for a 10% DEF consumption, it would have used 10 gallons of DEF.

Where do I put the diesel exhaust fluid?
Many manufacturers will add a small DEF tank to the vehicle. It can be located in the engine bay, or at the fuel filler cap area. If you are filling the tank up yourself, be careful to NOT add DEF to the fuel tank and vice versa.

How can I make my own DEF fluid?
This is not a good idea. Unless you have specialist equipment, I would advise against making your own diesel exhaust fluid. The process requires precision to achieve 32.5% urea content to 67.5% de-ionised water.

What automakers use or require diesel exhaust fluid?
There are a number of automakers that use or require it. Those include Volkswagen / VW, BMW, Mercedes Benz, Peugeot, GM / Opel, Ford, Toyota, Mazda, Ram, Nissan, and others. The best way to determine if your vehicle uses DEF is to consult the owners manual, and also use it to determine the DEF filler / tank location, so you can confirm.

DEF on retailer shelf

What is in BlueDEF?
BlueDEF appears to be essentially the same makeup as other DEFs. From the manufacturer website: “BlueDEF is a nontoxic solution of 67.5% purified water and 32.5% ultra pure automotive grade urea.”

What year did diesel exhaust fluid start?
The answer to that is “it depends.” It depends on the manufacturer and the engine. So on Ford pickups, the 6.4L Power Stroke, DEF has been required from April 2010 onward. For the new diesel F-150, 2018 and up. As for Chevy / GMC Duramax engines, it’s been required since 2011. Heavy duty Ram engines, DEF is required from 2013-on.

Can you put diesel in DEF tank?
Most of the times, the vehicle manufacturer will have the DEF tank filler neck away from the Diesel filler neck, to prevent mixing DEF and Diesel. Even if they are near each other, definitely do NOT put diesel in the DEF tank (or vice-versa).

Which cars use AdBlue, BlueDEF, etc.?
We are working on an article has more information on the different fluids available and specifications from engine/vehicle manufacturers. Stay tuned.

How do I find out the DEF fluid capacity in my truck?
This kind of information can be found in the vehicle owners manual. You may also find the information online by searching but the manual is the best resource, if available.

DEF Pricing / Availability

Where do I find DEF fluid reviews?
I would say the best place to find reviews in general is Amazon. Even if you don’t buy from there, Amazon is well-known for having user reviews on essentially everything.

What are the common commercial names for a DEF solution?
Some commercial names include BlueDEF, Prestone Command, Valvoline Premium Blue, NOxGuard, and AC Delco diesel exhaust emissions reduction fluid. There are others as well.

How much is DEF per gallon in jugs or at the pump?
It’s difficult to give you a definite price here. Pricing varies depending on your location and the brand and quantity. If you want to do a retail search of locations that carry the fluid, try DiscoverDEF. Alternately, you could try just Googling “DEF prices near me” or something similar.

How to clean DEF system?
Warm water is more than enough to clean the system.

What causes DEF to crystallize?
DEF contains Urea and water. Urea in its normal form is white crystals. When DEF loses its water content, Urea becomes solid again, that’s why you see the white crystallisation.

What is the ratio of DEF to diesel?
In terms of consumption, BlueDEF claims that one gallon of its product is good for 300 miles of driving. Please also see the question earlier in this article titled “How much DEF does a diesel use / consume?”.

How do you drain diesel exhaust fluid?
If you can access the DEF tank, there should be a drain plug at the bottom of the tank.

I hope you’ve found this article on diesel exhaust fluid helpful. I strive to provide authoritative, comprehensive information to diesel owners and enthusiasts.

Be sure to check out the other diesel related articles on this site. Also, consider submitting your email address at the bottom of the page so we can notify you when we publish new diesel articles.

If you have any questions or comments, please leave a comment below.

Automotive and Electronics Engineer – 6 years experience

Senior Technical Trainer on Diesel Engines – 6 years experience

Passionate about Teaching and New Technologies

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