5 Answers About Diesel Exhaust Fluid

With the addition of Selective Catalytic Reduction (SCR) to Cummins diesel engines for fire and emergency vehicles, we’ve answered some common questions about Diesel Exhaust Fluid (DEF). Here are answers to five frequently asked questions about DEF:

Q: Where can I find DEF?

A: Because almost all diesel-powered passenger cars and trucks built since 2010 are equipped with Selective Catalytic Reduction (SCR) and require Diesel Exhaust Fluid (DEF), it is readily available at most fueling stations. Truck stops often have a DEF pump right on the fuel island. You can also get DEF at major OEM locations, including Cummins dealers and distributors. If you have a large enough fleet and the storage capacity to justify bulk purchasing, your fuel jobber will probably be willing and able to supply DEF to you.

Cummins Filtration and Valvoline have teamed up to manufacture and distribute a high-quality Diesel Exhaust Fluid called Air Shield™ that meets exacting standards for quality assurance. To find the nearest Air Shield distributor or dealer, check out the retail locator at

For a comprehensive list of DEF retailers, visit There you can see local facilities that carry DEF, and even enter a trip and map out all sources for DEF along your route.

Q: What’s the shelf life of DEF?

A: That depends on the temperature of your storage facility. If you’re going to keep DEF in a climate-controlled environment at or below 77°F (25°C), it should keep its full potency for up to two years. If the temperature varies between 10°F and 90°F (-12°C – 32°C) or the DEF is exposed to sunlight, the shelf life will be reduced – but it should always last at least one year from the date code imprinted on the label. When tested at a constant temperature of 95°F (35°C) for more than six months, DEF meeting the IS0 specifications retained its correct formulation, so shelf life should not be an issue.

Q: What happens if DEF freezes?

A: Vehicle operation is not affected. During vehicle operation, SCR systems provide heat to the DEF tank and lines. The system thaws DEF rapidly and keeps it flowing to the aftertreatment system regardless of the outside temperature.

Diesel Exhaust Fluid is formulated with 32.5 percent urea and 67.5 percent deionized water. In storage or when the engine is not in use, DEF will freeze at 12°F (-11°C). At this concentration level, the urea and the water freeze and thaw at the same rate – ensuring that you always have the correct amount of each. Engines with SCR are specifically programmed for optimal performance at this ratio, which is why it is important to make sure you get a quality brand of DEF that meets ISO specifications.

Cummins Aftertreatment Systems are designed with heated lines and DEF tanks. In extremely cold conditions, they will rapidly thaw the DEF solution. There will be no delay in engine startup.

There is one operational difference to note: Like any water-based fluid, DEF expands when it is frozen (by approximately 7 percent). In cold weather, when the operator shuts the vehicle down, waiting 60 seconds before turning off the battery will allow the fluid to drain back out of the hoses and into the DEF tank.

No anti-gelling additive or freeze point improver should ever be added to the DEF, as they will impede its ability to perform correctly and may damage SCR system components.

Q: How much DEF will my equipment use?

A: If you use a fire truck in an urban setting, running 7,000 miles annually at 5 mpg, you can expect your DEF usage to be about 2.5 percent of diesel fuel consumption. In this scenario, you would use 1,400 gallons of diesel fuel and 35 gallons of DEF per year. The average DEF tank holds five gallons of fluid, so you would need about seven DEF tank fills per year (one every 7.5 weeks).

Since most DEF fill-ups will take place at the same time as you are getting diesel fuel, it’s helpful to look at usage from that perspective. Assuming that you have a 65-gallon fuel tank and DEF usage is 2.5 percent, you’ll use 1.6 gallons of DEF for every tank of diesel fuel. With an average DEF storage of five gallons, you’ll need to replenish DEF only every fourth time you refill with diesel fuel. Of course, our recommendation is to simply top off your DEF tank each time you refuel. If you are running a vehicle that gets very low actual activity, or the vehicle is stored in a barn with high temperatures, where shelf life might be a concern, adjust your DEF refills accordingly and consider having a spare bottle of DEF available in the event of a prolonged emergency call.

Q: What happens if my equipment runs out of DEF?

A: All EPA 2010 engines with SCR are designed with a gauge that shows the DEF fluid level, similar to a fuel gauge. In addition, they are equipped with a system of flashing lights to alert the operator well in advance when the DEF tank is getting low on fluid. If the reservoir is not replenished with DEF and runs low, vehicle speed will be limited to 55 mph, but as soon as DEF is added, the engine will resume normal speed levels. In the unlikely event that the DEF tank runs dry, the vehicle speed will be limited to 55 mph, and the engine will continue to support pump operations until it is keyed off. Once it is keyed off, vehicle speed will be limited to 25 mph, and pumping may be limited. Once the DEF tank is refilled, normal engine operations will be restored. A prudent measure would be to have a top-off gallon jug of DEF available on each piece of equipment with an EPA 2010 engine with an aftertreatment system.

Engines manufactured before July 8, 2011, may behave differently than described above. Talk to your local Cummins representative for more details, and ask for Cummins Bulletin 4971316, “Driver Tips For Fire And Emergency Vehicles” [PDF 315 KB].

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