Car Oils And Fluids

What Happens When You Run Out Of Diesel Exhaust Fluid (DEF)?

Mike Cross
Updated Feb 22, 2022
What Happens When You Run Out Of Diesel Exhaust Fluid

If you own a diesel truck or vehicle, then not only do you have to worry about keeping diesel in it for it to run, but you also have to stay on top of the diesel exhaust fluid.

It is essential to keep the liquid filled because it has an important job and the consequences of not keeping it supplied could leave you in a rough spot.

Read on to learn about what the diesel exhaust fluid does and also what happens if you run out of diesel exhaust fluid in your diesel truck.

What Happens When You Run Out Of Diesel Exhaust Fluid
What Happens When You Run Out Of Diesel Exhaust Fluid


What does it do when you run out of Diesel Exhaust Fluid (DEF)?

First before getting into what happens if you run out of diesel exhaust fluid, we will get into what it is and what it does.

To start with diesel exhaust fluid is a two to one mixture of pure urea and water.

It is 32.5 percent pure urea and 67.5 percent de-ionized water. This mixture is a blue color and goes into the diesel exhaust fluid tank. It is essential to make sure to fill the right tank when adding it in or else it will not work right and could damage other parts of your car.

Note: Don’t confuse your diesel exhaust fluid with your windshield wiper fluid or anti-freeze.

Next, the purpose of the diesel exhaust fluid is to mix with the emissions from the burning diesel from the engine, so it will not release bad gasses into the air.

The way this works is when the diesel is burned, the smoke from it burning goes into a chamber and the diesel exhaust fluid is released into that chamber as well. Inside the exhaust pipe, the diesel exhaust fluid is vaporized and decomposes into ammonia and carbon dioxide.

Then it travels into the selective catalytic reduction and combines with oxygen and detrimental nitrogen oxides. This produces nitrogen, water vapor, and carbon dioxide which finally comes out of the exhaust pipe. This keeps the environment safe and the air clean.

How long does Diesel Exhaust Fluid Last?

How long does Diesel Exhaust Fluid Last
How long does Diesel Exhaust Fluid Last

It is crucial to buy a diesel exhaust fluid that complies with the International Organization for Standardization 22241 specifications. This will make sure that it is the right stuff and your vehicle will run right. As for how much to buy at one time, you can buy any amount because it does not deteriorate or break down.

The one thing to remember though is to store it correctly because it can freeze since it has so much water content. Make sure to store it in a heated area to prevent this.

Also, you don’t have to shake the fluid before using it or mix it in any way because the ingredients remain in solution even when sitting on the shelf.

What Happens if Your Diesel Exhaust Fluid Runs Out

So now that you know what diesel exhaust fluid does and how to store it we can get into what happens when it runs out and how to prevent it from running out.

First, before getting into what happens, we will look at how to prevent it from running out.

One of the best ways to avoid your diesel exhaust fluid from running out is to top off the tank every time you fill up or every other time you fill up the diesel in your vehicle. This works because a lot of times the manufacturer will make the diesel exhaust fluid tank the same size as the diesel tank. That way it is an easy reminder for you to fill the tank up

If you aren’t in an area when you fill the diesel tank up to fill up the diesel exhaust tank, it is okay though because all diesel vehicles have some warning system to let you know when the fluid is starting to get low.

Some manufacturers put a diesel exhaust fluid gauge on the vehicle just like a gas gauge only, and it is for the diesel exhaust fluid.

Others have warning messages that trigger. On newer diesel vehicles with the computers and screens built in, messages will start to show up there when the tank gets low.

There also might be a setting to check the level of the tank on the computer screen even if there is not a gauge, just like you can check oil life.

Diesel vehicles give you plenty of warning to let you know your exhaust fluid is getting low before you run out completely.

The reason they give you ample warning is that if you run out of diesel exhaust fluid completely, then the consequences can leave you stranded. It can be just like you ran out of diesel gas in your truck.

In most vehicles, before you run out completely, the diesel truck will start running on a lower power mood. This does two things. One it makes the fluid that you have remaining last longer because you won’t be able to burn it as fast and two it becomes annoying, so you will be sure to refill it as soon as possible. If you keep driving it on a lower power mood and it runs out, then the major consequences happen.

If your diesel exhaust fluid runs out entirely, then the vehicle will not start at all. A lot of times a warning message will pop up, that says the vehicle is nearing the not starting point. If you see this message, then be sure to stop in an area where you can get diesel exhaust fluid to fill the tank or else you will be stranded.

The good thing is, as long as the vehicle is running, even if you run out of fluid. It will stay running. This is because you don’t want the truck to stop while driving, but the environmental protection agency is tough on making sure you have fluid to follow the laws.


Now you know the importance of diesel exhaust fluid. Luckily, diesel exhaust fluid can be obtained at most stores and auto shops, so it is easy to get in a pinch

Also, it stays good in-defiantly, so you can stock up ahead of time as long as you don’t let it freeze.

It is important to keep fluid in the tank because if you don’t, your vehicle will not run at full power or worse will not start at all.

The trick to helping you not run out is to refill the diesel exhaust fluid every time you fill up the diesel tank.

If you can’t do that though, then pay attention to the warnings that the vehicle provides and when you see them or the warning light, then refill the diesel exhaust fluid at the next chance you get.

Mike Cross
Life is too short to drive with stock audio

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9 comments on “What Happens When You Run Out Of Diesel Exhaust Fluid (DEF)?”

  1. Always take care of that diesel. I have learned to make notes and routing checklist through my travels. Thanks for sharing!

  2. I found it interesting that you suggest topping off your diesel exhaust fluid every other time you fill up your tank. My dad just got a truck and wants to be able to take care of it well. I will send him this information so he can find a good place to go to get his supplies.

    1. What happened if I don’t run my truck this winter ?and what should I do? Thank you very much

  3. Don't want your diesel to run into a mood LOL.

    Pretty sure the DEF tank is not the same size as the fuel tank, but it may be proportional so that they run out about the same time

  4. Thanks for mentioning that diesel exhaust fluid should be topped off whenever you are filling up the diesel in your vehicle. I own a tractor that runs on diesel and I am worried that I will run out of exhaust fluid in a place that I won't be able to refill it. I'll make sure to top off the fluid whenever I am able to.

  5. Fords deisel tech says I have deisel fuel in my DEF tank and DEF system is contaminated and all parts need replacing. How did fuel get into the DEF system. Tech replaced DEF pump could it have happen then by replacing DEF pump?

  6. 2011 F250 SD KR, I often get a contaminated DEF warning every 200 or 2K miles. Sometimes it clears on its own other times I pump it out so it can clear any reason as to why it gives this warning?

  7. Urea is a component of human urine. Turns out, DEF is 1/3 urea, 2/3 distilled water. In simpler terms, it's just DILUTED PISS. Next time you run out, just pee in the DEF container, add the appropriate amount of distilled water and away you go. For free. You're welcome.

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