What is DEF?

A little DEF cleans a lot of air.

Commonly referred to as DEF, diesel exhaust fluid is an emissions after treatment, introduced to the exhaust gas of a diesel engine to reduced nitrogen-oxide (NOx) emissions.

You may have seen DEF (also known as AdBlue, which is a brand name) in the aisles of a truck stop or hardware store. The fluid, which is comprised mostly of urea and water, is typically sold in 1- and 2.5-gallon containers, as well as larger vessels for use in over-the-road trucks. DEF is also retailed at truck-stop fuel pumps in bulk form, which is convenient for truck operators, as big tractor trailers often have DEF tanks as large as 20 gallons. Diesel-powered pickup trucks have smaller DEF tanks, typically 5-6 gallons.

Emissions

Crucial to the operation of what is known as the Selective Catalytic Reduction (SCR) system, DEF employed correctly can help to reduce NOx emissions by as much as 90 percent. One of the most dangerous byproducts of diesel combustion, NOx emissions contribute to respiratory problems, acid rain, and ground-level ozone, also known as smog. Environmental Protection Agency (EPA) regulations mandating the use of DEF were passed into law in 2010, requiring that all heavy-duty diesel engines built beginning in 2015 employ the SCR system.

DEF Packaging and Prices

The fluid itself, which is about two thirds water and one third synthetic urea, is not especially expensive, and can run from $8-$12 per gallon, depending on the retailer and package size. The pump-side dispensers tend to be the least-expensive purchase option. A 20-gallon DEF tank will provide a typical over-the-road truck with about 6000 miles of operation.

Limp Model

By law, once a vehicle’s DEF talk is depleted, the truck in question will operate in “limp” mode, which reduces engine power output until fluid is added to the tank. A warning that the DEF tank is approaching empty typically appears on a truck’s instrument panel approximately 100 miles in advance of complete depletion.

Tampering

While it was briefly popular for the operators of diesel-powered trucks to remove the SCR systems, that action has proven detrimental to long-term ownership of the vehicle. Not only is tampering with the SCR system illegal, many dealerships will decline to take an obviously tampered-with truck in trade.

Vehicles

Truck requiring DEF use include diesel-powered heady-duty versions of the Chevrolet Silverado, Ford F-Series pickup, GMC Sierra, and Ram pickup. These in addition to all diesel-powered medium- and heavy-duty trucks.

Handle with Care

Dried DEF tends to crystalize on vehicle surfaces, so care should be exercised when adding the fluid to your vehicle. Left to dry, DEF may fade painted body panels.