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What is DEF?

Diesel exhaust fluid (DEF) reduces nitrogen oxide emissions in diesel engines to meet stricter EPA standards passed in 2009.

DEF stands for Diesel Exhaust Fluid. It goes into a special tank (separate from the fuel tank) on diesel vehicles made after 2009 to reduce pollution in the exhaust from the engine. Newer diesel cars and trucks use a system that sprays diesel exhaust fluid into the exhaust fumes to break down the nitrogen oxides in them. After the exhaust passes through a special converter, what’s left of the exhaust is mostly harmless water and nitrogen. The DEF system allows manufacturers to meet emissions standards while still making diesel engines with as much power and gas mileage as they did before the pollution standards were made more strict.

While DEF allows your engine to have more power and gas mileage, the downside is that you will have to keep the tank full.

What happens if I let my truck run out of DEF?

Depending on the truck, it will either shut down or greatly reduce the engine power until you refill the DEF tank. The EPA requires that trucks that use diesel exhaust fluid also have a system in place that will warn you when your DEF is low, and that will prevent you from running the truck without diesel exhaust fluid.

Luckily, it’s pretty hard to let your truck run out of DEF. When it’s getting low, you will see a light on your dashboard telling you so. Fill up immediately when you see the first light, because if you ignore it for too long, the emissions system will start to cut your engine power.

How often do I need to refill my DEF tank?

How often you will need to fill up your DEF tank depends on how much your tank holds, how much you drive, and the rate that your truck uses DEF. Check your car or truck’s owner’s manual to see what the manufacturer recommends. If you don’t find anything there, a good rule of thumb is to regularly top it off with every other oil change.


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