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4 Myths About DEF Fluid Busted

A diesel fuel and DEF fluid filler cap.

Owning a diesel car most likely means getting familiar with DEF fluid, which is used as part of the emissions system to keep your tailpipe as clean as possible. Since diesel exhaust fluid needs to be refilled on a fairly regular basis, you’ll likely encounter a lot of “folk wisdom” and “expert advice” about where you can buy the fluid, who can pour it into your car and what effects certain brands might have on your vehicle’s performance and warranty.

Let’s take a look at the facts, not the fictions, surrounding diesel exhaust fluid.

1. You Can Only Buy DEF Fluid at a Dealer — NOPE

When you bought your car, you may have been told by a salesperson that the dealership was the only place you would be able to purchase DEF, and that you better stock up right away for the sake of convenience. The thing is, just like almost any other automotive fluid — oil, washer fluid, coolant — DEF is available at your local NAPA Auto Parts store, as well as at many fuel filling stations and truck stops. Essentially, wherever you can buy diesel fuel you can also buy DEF, so don’t feel pressured into paying that dealer mark-up.

2. Only a Dealer Service Tech Can Refill Your DEF Fluid — NOT TRUE

When buying DEF at the dealership, you may also be told that you need to pay to have a certified technician add it to your car’s DEF tank. This couldn’t be further from the truth. Almost all modern cars place the filler for diesel exhaust fluid immediately beside the fuel filler, with a few exceptions – some relocate it under the hood or in the trunk. If you are comfortable adding washer fluid to your car, you won’t have any issue pouring DEF in either, and you certainly don’t need to pay a dealer for this 60-second job.

3. Using Non-Dealer DEF Fluid Voids Your Warranty — FALSE

DEF container

It’s not legal for a dealership to insist you only purchase lubricants, fluids and certain other types of components from them under threat of having your warranty voided. The flip side of that, however, is that you need to be able to prove that you have been following the recommended DEF refill schedule should any issues with your car crop up in the future. This means keeping receipts from each bottle purchased and, ideally, a log of the date and mileage for each top-up.

4. Non-Factory DEF Fluid Will Hurt Performance — DEFINITELY NOT

DEF is just a fancy name for a mixture of water and a chemical called urea. There’s really no magic behind it, which means that any brand of diesel exhaust fluid is going to work just fine in your car. There’s zero chance that not using dealership-supplied DEF will in any way impact engine power, fuel mileage or overall performance.

Check out all the chemical products available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on DEF fluid, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Benjamin Hunting View All

Having been bitten by the car bug at a young age, I spent my formative years surrounded by Studebakers at car shows across Quebec and the northeastern United States. Over ten years of racing, restoring, and obsessing over automobiles lead me to balance science writing and automotive journalism full time.  I currently contribute as an editor to several online and print automotive publications, and I also write and consult for the pharmaceutical and medical device industry.

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