If you drive a modern diesel, you’ve probably added DEF fluid—but what is it really, and why does your truck depend on it?
In this Hidden in Plain Sight episode, we break down Diesel Exhaust Fluid (DEF), a simple mixture of 67.5% deionized water and 32.5% urea. Yes—urea, the same compound found in urine. Humans and many mammals, such as cows, dogs, and deer, naturally produce it, though DEF uses a clean, synthetically manufactured version made from ammonia and carbon dioxide.
DEF is injected into your exhaust system (not the engine), where it helps convert harmful nitrogen oxide (NOx) emissions into harmless nitrogen and water vapor through a process called Selective Catalytic Reduction (SCR). The result is up to a 90% reduction in pollution.
We also tackle a common concern: does DEF cause hesitation or loss of power? The answer—no, not directly. But modern diesel emissions systems are complex. If something fails, your truck may enter “derate mode,” intentionally limiting power. That’s often what drivers are feeling—not DEF itself.
We’ll cover how long DEF has been in use, how it’s made, and why your truck won’t run properly without it.
And just to clear things up—DEF is not fuel, not an additive, and definitely not a tribute band for Def Leppard.
It’s one of the most overlooked systems on modern diesel equipment—and one of the most important.
Take a closer look at the ordinary things around you… and discover the extraordinary stories hiding in plain sight.
👉 Watch the full Hidden in Plain Sight playlist for more episodes like this.
AI Generated·May contain errors
Hey, Dear Farmer Jay here. If you drive a modern diesel vehicle, you've added DEF,
which is Diesel Exhaust Fluid, to a reservoir on your vehicle. But what's it made of?
What's it do? And why is it necessary for today's diesel vehicles?
Well, a couple of things right off. This is not a fuel additive. If you look at the packaging of
this brand right on the top, it right there says, do not add to diesel fuel tank or mix
with other products. This is not a fuel additive. It actually goes into the system
that treats the exhaust that comes out of the engine after combustion. So it's not an additive.
Secondly, you need to know what it's made of. And right here on the back of the box,
it'll tell you, you can see right here, ingredients, 67.5% water, deionized water,
actually, and 32.5% urea. Urea? Yep, urea. Now, did you know that all mammals, like us,
like deer, like cattle, horses, and so forth, produce urea in their urine? Therefore,
the similar sounding names. Now, lest you start walking by your diesel vehicle and giving the
stink eye at the place where this is put in, notice that the concentration of this was quite high.
It was 32.5%, whereas the average urea content in human urine is about 2%. How it's done is by
synthetic manufacturing. Ammonia is subjected to carbon dioxide under high pressure and heat
to produce synthesized urea in higher concentrations. Then that's put into this
product with water. And you go, okay, sounds like a waste of time and money. Well, diesel,
whether we like it or not, is a dirty fuel. It burns, you've seen it, different clouds coming
out of either regular light pickup trucks, medium-sized pickup trucks, or large diesels
where someone really romps on it, or heavy equipment, and you get a black cloud that
comes out. Well, by using this in the system, this will reduce up to 90% of that. And here's
how it's done. The DEF is injected into your exhaust system, not the engine, where it helps
convert harmful nitrogen oxide emissions into harmless nitrogen and water vapor through a
called selective catalytic reduction. And the result is, as we said, up to 90% reduction
in particulates and pollution into the air. Even though it's been around since the early 1900s for
industrial use, since 2010, it's become commonplace and usually required on the modern diesel engine.
And for good reason, it's an important system, keeps it running well and reduces pollution.
If you like finding out items like this that are hidden in plain sight,
they're just things that always make you go, I wonder what the story is behind that.
Check out this playlist, our hidden in plain sight. You'll walk away with all sorts of
answers to things that you've always wondered about, but never took the time to take a look.
Hey, until the next time, this is Dirt Farmer Jay at DirtFarmerJay.com.
