What Happens if You Run Out of DEF?

Like any other engine fluid, diesel exhaust fluid (DEF) is essential to keeping your vehicle up and running. It’s a mixture of water and urea that gets injected into your exhaust before it exits the tailpipe, causing a chemical reaction that transforms harmful pollutants into clean air.

DEF was introduced in 2010 when the Environmental Protection Agency (EPA) tightened its emission standards on diesel machines. It is part of the Selective Catalytic Reduction (SCR) system, which drastically reduces the amount of pollution emitted from vehicles. Today, any new diesel machine above 74 horsepower is required to have an SCR system and use DEF.

Exhaust is injected with DEF before entering the SCR catalyst, where it is then transformed into clean air that exits the tailpipe.

If you let the DEF tank run dry, you could experience days, weeks or even months of downtime — along with thousands of dollars in potential repairs. We crunched the numbers in T3, the operating system for construction, and found that one in 10 machines can run out of DEF every day.

In reality, you should never run out of DEF. Your vehicle displays a warning light on the dashboard when its tank is low, and if you use T3’s real-time maintenance alerts, you can receive a text message or email that tells you which vehicle in your fleet needs DEF and where it is located.

But as the data shows, mistakes happen pretty often — and sometimes these mistakes can cause costly challenges. ‍

The Cost of Running Out of DEF‍

In a best-case scenario, running out of DEF simply means refilling the tank and restarting your machine. If you have DEF nearby, you could be back to work within minutes.

But sometimes it isn’t so quick and easy. For example, your DEF header — the sensor system that reads the level, temperature and quality of your fluid — can break when the DEF tank runs dry. Replacing this part can cost up to $1,200, and your downtime will vary depending on parts availability. 

The Sany Gen 6 DEF Header, available on shop.equipmentshare.com

Over the long run, repeatedly running out of DEF can also cause your vehicle’s exhaust system to clog with soot and debris, leading it to run less efficiently and eventually break down. Depending on the severity of your breakdown, it could take weeks to clean out your exhaust system and replace any clogged or broken parts. Plus, the service calls alone could run you thousands of dollars in time and labor.‍

How Often Do You Need To Refill the DEF Tank? ‍

The EPA has established minimum refill intervals for light-duty and heavy-duty vehicles, but a good rule of thumb is to top off your DEF tank every time you refuel your machinery. This might sound like overkill — but there’s no such thing as refilling the tank too often. DEF can be purchased at most auto parts stores, hardware stores, truck stops and also at gas stations. Be sure to store it in a cool and dry environment, as it will degrade if left in direct sunlight.

The DEF tank is often located right next to the diesel tank. You can tell the difference by looking at the color of the caps: Typically, the DEF tank has a blue cap, and the diesel tank has a green cap. Always double check to make sure you put the proper fluid inside the proper tank. Otherwise, you risk causing major damage to your vehicle. We’ve seen some service tickets surpass $20,000 for this very reason. 

All told, DEF is just as important to your vehicle as fuel, coolant, oil and all the other fluids you regularly check and replace. Your machine will not function without DEF, and you can face serious consequences from letting the DEF tank run dry. 

Learn more about T3's real-time maintenance alerts