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I Accidentally Put DEF Fluid in a Fuel Tank. Now What?

Four nozzles at a fuel pump

Nobody’s perfect, so accidents are bound to happen. You’re not the first person to put DEF fluid in a fuel tank, and you won’t be the last. Here’s what to do to fix this situation and how to prevent it from happening next time.

What Is DEF?Side-by-side diesel fuel/DEF fillers

DEF stands for diesel exhaust fluid. If you’ve been driving diesels for years and haven’t heard of it, that’s because it’s relatively new. Starting in 2010, the Environmental Protection Agency required all new diesel engines to emit less nitrous oxide. One way manufacturers can meet that standard is to use selective catalytic reduction.

Here’s how that works: Exhaust gas goes through a filter that traps the really grimy stuff like ash and soot. The exhaust gas that gets through the filter is sprayed with DEF, which is 67.5% purified water and 32.5% urea, and goes into the catalytic converter. The catalytic converter uses various metallics to clean the pollutants further before exhaust exits the tailpipe. Not only does the diesel vehicle run cleaner, it also burns fuel more efficiently, which translates to better fuel economy.

Therefore, DEF is good. DEF fluid in a fuel tank, however, is bad.

What Happens When You Put DEF in Your Fuel Tank?

If you pour DEF into your fuel tank and realize your mistake immediately, your fuel system needs to be flushed, and there may be some part repairs or replacements required just from the fluid having been there.

If you don’t realize your mistake immediately and start your engine, you’ll realize your mistake almost immediately. Your engine will stop running, and there will be more work replacing or repairing parts before you get back on the road again.

Basically, this is a situation where you need to take your vehicle to a professional, so they can assess the damage and do any necessary maintenance.

How Do You Avoid Doing This?

Here’s how to avoid putting DEF in your fuel tank in the first place:

  • DEF fillers have blue caps, which are smaller than your fuel tank cap.
  • Some manufacturers put DEF fillers right next to the fuel filler. Again, they’re smaller and blue, which should make it easy to tell the two apart.
  • Some manufacturers put the DEF filler under the hood. The upside of this is that the filler is not right next to the fuel filler. The downside is that if you’re not familiar with what’s under your hood, you might think you’re supposed to add DEF to the fuel tank.

DEF is a great automotive advance — it leads to cleaner diesel emissions and better fuel economy. Just be sure to keep it in the right tank.

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

Photos courtesy of Pixabay and Mike Hagerty.


Mike Hagerty View All

Mike Hagerty is an automotive journalist whose work has been featured on radio, TV, in print and online since 1997. He's the Publisher and Editor of, and contributes car reviews to the Los Altos Town Crier and Previous outlets have included KFBK and in Sacramento, California, the ABC television affiliates and Hearst-Argyle and Emmis radio stations in Phoenix, Arizona; AAA magazines for Arizona, Oklahoma, Northwest Ohio, South Dakota and the Mountain West and

2 thoughts on “I Accidentally Put DEF Fluid in a Fuel Tank. Now What? Leave a comment

  1. I put $4 worth of DEF in my diesel tank after I put $85 worth for diesel in my fuel tank if anything I need to know is anything going to happen to my truck

    • Stop! Don’t start or drive the vehicle. There is no “safe” amount of DEF allowable in diesel fuel. The fuel system must be flushed completely and inspected, as well as all the fuel filters replaced. Your local NAPA Auto Care can handle this process if you are unable to DIY.

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