Diesel Exhaust Fluid (DEF) plays an important role in moderating truck emissions and meeting EPA guidelines. As with any type of vehicle fluid, regular monitoring of DEF levels is an important part of ongoing maintenance. While checking diesel exhaust fluid levels isn’t a difficult job, there are factors that can complicate the issue. Here’s a quick overview of everything you need to know.


DEF Basics

Diesel exhaust fluid is required for a process called selective catalytic reduction (SCR). SCR works by diverting exhaust gases through a filter and then into a catalyst. There, the gases mix with DEF, which turns hazardous NOx emissions into a harmless mixture of nitrogen and water.

Diesel exhaust fluid has to be replaced on a regular basis. Replacement intervals are tied to the fuel efficiency of your engine. Less efficient engines produce more exhaust gases, which require more DEF to treat.

Typically, dose rates for a well-maintained heavy- or medium-duty truck engine will be between 2-3%. This means that a truck getting six mpg will require between three to five gallons of DEF for every 1,000 miles it travels.

How to Find What You’re Looking For

All truck engines meeting 2010 or later EPA guidelines are required to have a dashboard warning system that alerts you when DEF levels start running low. These systems feature an amber warning light that goes off when levels drop below 10%. At 5%, the light starts flashing, and when levels drop below 2.5%, it turns solid. When DEF runs out entirely, the light turns red, and the truck is electronically limited from going more than five mph.

DEF levels can also be checked manually. The storage tank can typically be found next to the diesel fuel saddle tank on a truck, or in the spare tire area of a passenger vehicle.

Topping Off

Replacing DEF is fast and easy — even if a level check doesn’t indicate that you’re running low, it’s always best to fill up before a long haul and avoid the risk of a problem. DEF can be found in all truck stops and auto parts stores. It is a non-toxic liquid that can be handled without any special protective clothing or tools.

To add more DEF, simply locate the fill port — it is usually found under the hood, next to the fuel port or in the trunk — and top it off manually. Fuel and DEF ports are differentiated in size to reduce the risk of accidentally putting one into the other.

How Long Does Diesel Fluid Last?

Diesel exhaust fluid normally has a shelf life of about two years. However, exposure to sunlight or sustained high temperatures can compromise it. Any DEF purchased at a truck stop or auto parts store will have a clearly labeled expiry date — using DEF past this date can cause performance issues and potential maintenance problems.

Calculating DEF Requirements

All newer diesel vehicles have a dashboard warning system that alerts you when it’s time to change your DEF. To estimate how much you’ll need, you’ll need to know the efficiency of your engine.

DEF is consumed at a rate of about 2-3% relative to the amount of fuel you are using. This means between 1.2 and 2.0 gallons of DEF will be required for a vehicle with a 65-gallon gas tank. If you have a five-gallon DEF tank, DEF should be replaced every third or fourth time you fill up. However, the easiest way to avoid a problem is simply to top off periodically.

DEF Hazards

Because DEF is mostly water, there is a risk that it can evaporate or freeze. Evaporation occurs at high temperatures, but preventing it is easy — simply keep unused DEF in a tightly closed container. Within the tank itself, there is no risk of contamination.

DEF can freeze at temperatures below 12°F. However, manufacturers have included a number of safeguards to prevent damage when this occurs. Trucks can still be run normally with frozen DEF — most tanks feature heating elements that will automatically warm it over the course of normal operation.

DEF that has been previously frozen won’t be compromised. Water and urea freeze at the same rate, so the basic ratio won’t be affected. However, frozen DEF does expand, typically by about 7%. To prevent damage to your SCR system in cold weather, give fluids a minute to drain back into the tank before shutting down your vehicle’s battery.

Contact us for more information about keeping your DEF levels where they should be.

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Post updated on January 14th, 2019