General information only, last reviewed October 25, 2026. Full notice and dates to watch at the end of the page. A date on this page has passed. Check the current rule at the source before you rely on it. Dates to watch
Pull up to a modern truck stop and you will see a second nozzle next to the diesel: DEF. It is a fluid that lets today's diesel engines run clean, and trucks need it everywhere they fuel. For a jobber serving fleets, that makes it a natural product to carry. This covers what DEF is, how it works, and where it fits in the business.
What DEF is
DEF, diesel exhaust fluid, is a non-hazardous, colorless solution of 32.5% urea and 67.5% deionized water. Modern diesel engines use it to reduce emissions. It lives in its own tank, separate from the diesel, and is injected into the exhaust to clean it. It is not a fuel, and you never mix it into the diesel tank.
How it works: SCR
DEF is the working fluid of a selective catalytic reduction (SCR) system. It is sprayed into the hot exhaust, where the urea breaks down into ammonia. That ammonia reacts with nitrogen oxides (NOx) over a catalyst and turns them into harmless nitrogen and water vapor. Because it all happens after combustion, it cleans the exhaust without costing engine performance or fuel economy.
Why fleets depend on it
Trucks with SCR systems need DEF to run legally and cleanly, and an engine that runs out will derate or stop. So for a fleet, DEF is a steady operating need right alongside diesel, which is why cardlock and fueling sites increasingly offer it at the island.
How jobbers handle it
Jobbers treat DEF as another product line: bulk delivery to tanks, jugs and drums for smaller users, and dispensers at fueling sites. Since trucks need it wherever they fuel, offering DEF next to diesel is an easy add for a jobber serving fleet customers. It is one more product to price, track, and invoice, which is why FastDragon Fuel Jobber carries DEF as a tracked product beside the fuel. For what a DEF route has to record, tote by tote, see DEF delivery software.
Handling is where DEF gets picky
DEF freezes at 12°F. The 32.5% urea blend was chosen because that exact ratio has the lowest freeze point of any urea-water mix, and freezing does no harm once it thaws. Heat is the bigger enemy: storage above 86°F degrades the urea and shortens shelf life, so bulk tanks belong out of the sun. It also demands dedicated tanks and dispensing equipment, as API's handling guidance for the ISO 22241 standard spells out, because trace contamination, even from tap water, can damage an SCR system. Like fuel, DEF is a volume product with a modest per-gallon margin, so tight handling and accurate billing decide whether the line pays.
Common questions
How much DEF does a truck use?
Cummins puts DEF consumption at roughly 3 to 5 percent of diesel consumption for its engines, so a truck that burns 100 gallons of diesel uses about 3 to 5 gallons of DEF. Dosing varies with load and exhaust temperature. For a fleet, that ratio makes DEF demand easy to forecast straight from fuel volume.
What happens if a truck runs out of DEF?
The truck does not stop mid-highway. EPA certification rules and guidance require an escalating inducement: warnings first, then reduced power or speed. On older engine software that can end in a crawl of about 5 mph if the tank runs dry. Engines on EPA's newer schedule (2027 and later heavy-duty engines, and older ones updated under EPA's 2025 guidance) step down over many hours of driving to 25 mph. A July 2026 EPA proposal would drop these derates from new engines; it is still only a proposal.In July 2026 EPA proposed dropping these derates from new engines; check EPA for whether that rule is final. Refilling restores normal operation, but the derate is disruptive enough that fleets treat an empty DEF tank like a breakdown.
How long does DEF last in storage?
API, which runs the DEF certification program, puts shelf life at about one year when stored at or below 86°F, 18 months at or below 77°F, and three years at or below 50°F. Sustained storage above 86°F can cut shelf life to around six months as the urea degrades, and DEF kept above 95°F should be tested before use. Buy in quantities you will actually turn, keep bulk tanks out of the sun, and check the batch date on packaged product.
Does DEF go in the diesel tank?
No. DEF has its own fill with a blue cap and a smaller nozzle, and it must never mix with the fuel. If DEF ends up in the diesel tank, or diesel in the DEF tank, do not start the engine; the tank needs to be drained, because running it spreads the damage through the fuel system or the SCR system.
Sources: EPA's diesel exhaust fluid page, its August 2025 inducement guidance, the SCR inducement rule for 2027 and later engines at 40 CFR 1036.111, and the July 2026 proposed rule; API's DEF storage and handling fact sheet and DEF consumer guidance; and Cummins on DEF consumption.