If you’re googling this topic, accidentally putting DEF in your diesel tank, you’re having one of those days. Rest assured that you’re not alone. However, you can’t ignore or hide this oopsie. Pull yourself together, take action, and avoid making a bad situation worse.

If you’ve accidentally put Diesel Exhaust Fluid (DEF) into your diesel fuel tank, it’s important to act quickly, as this blunder can cause serious damage to your diesel truck’s engine and fuel system. Even if it’s “just a few gallons” in a mostly full diesel tank, it’s not something you can ignore or wish away after a couple miles of running roughly. While both diesel fuel and DEF are essential to the operation of modern diesel engines, they serve very different purposes and should never be mixed.

What Is Diesel Exhaust Fluid? What Is DEF Used For?

Diesel Exhaust Fluid (DEF) is a non-toxic, non-combustible colorless liquid made of 32.5% urea and 67.5% deionized water. It’s designed to be injected into the exhaust system (not the engine itself) of diesels equipped with Selective Catalytic Reduction (SCR) systems, where it reacts with nitrogen oxide (NOx) to form harmless nitrogen and water vapor.

In other words: DEF does not go into the fuel system. It belongs in a separate DEF tank, which is clearly labeled and typically has a blue cap. Diesel fuel and DEF serve completely different functions and should never mix. Diesel fuel (obviously) powers the engine. It goes into the diesel fuel tank and is ignited in the combustion chamber to produce energy. So, while your truck consumes both liquids, they have completely different roles, chemical properties, and storage systems.

What Happens If I Accidentally Put DEF In My Diesel Truck’s Fuel Tank?

If you put DEF in the diesel tank, you’re effectively contaminating the fuel system with a water-based solution that does not combust and does not compress. While diesel engines tolerantly burn various diesel fuels, they are not designed to tolerate water in the fuel system.

When DEF mixes with diesel fuel, it results in costly repairs that may necessitate a complete engine replacement:

• Corrosion of metal parts (fuel injectors, fuel pumps, lines, tank)
• Engine failure
• Injector and pump failure
• Engine misfiring or shutdown
• Hydrolock, a situation where water prevents the piston from moving, potentially bending internal components and causing catastrophic engine damage
• Clogging of fuel filters and injectors

What If It’s Only A Few Gallons Of DEF?

Even a small amount of DEF can compromise the fuel’s lubricating properties, and once the engine runs, that mixture begins damaging internal components almost immediately. Just a few gallons (or even less) can contaminate the entire fuel supply. DEF doesn’t dilute harmlessly into diesel. Instead, it creates a harmful mix that can destroy your fuel system. The damage can cost thousands of dollars to repair, especially if the engine has been started and run with DEF-contaminated fuel.

Can I Still Drive My Truck?

Absolutely not; don’t try. If you realize you’ve put DEF into the diesel tank—which is the best-case scenario at this point—do not start the engine. Turning the key will circulate the contaminated fuel throughout the system and significantly increase the repair bill. Driving the vehicle even a short distance can turn a flush-and-clean job into a full-blown engine rebuild.

What Should I Do If I Put DEF In The Diesel Tank?

1. Do not start the engine.
   The moment you realize the mistake, stop. As mentioned, starting the engine will circulate the DEF throughout the system, making the damage worse.
2. Call a professional mechanic or towing service.
   The vehicle will need to be towed to a repair facility. A technician will have to drain the fuel tank, flush the entire fuel system, replace fuel filters, inspect the injectors and fuel pump for damage, and possibly replace affected components.
3. Be honest with the shop.
   Tell them exactly what happened. This will help them diagnose and fix the issue more efficiently and prevent further damage.

Can This Mistake Be Fixed?

Putting DEF in your diesel tank is a serious but surprisingly common mistake. Don’t panic, and definitely disclose the situation to someone in-the-know. If you catch it early and avoid starting the engine, you can minimize the damage. Obviously, prevention is key, so always double-check the caps before refueling or topping-off fluids—diesel in the fuel tank, DEF in the DEF tank.

Your secret is safe with us. We won’t tell anyone what happened, but trust us—you need to tell someone.