Contaminated DEF: Signs and Solutions

Close up image of an illuminated check engine light on a vehicle's dashboard

As much as we’d like to deny it, mistakes can happen to anyone, and DEF system contamination is always just an accident away. Even experienced drivers and mechanics have found themselves face to face with a contaminated fuel tank… and this kind of incident can send a good truck straight to the scrapyard.

Defining DEF

DEF, or diesel exhaust fluid, was developed to make diesel engines run cleaner. DEF is made up of 32.5 percent synthetic urea and 67.5 percent deionized water, and DEF systems can be found in diesel cars and trucks manufactured from 2010 and beyond. Most trucks have a DEF heating system to keep them running in extremely cold climates.

In order to reduce emissions, DEF systems utilize selective catalytic reduction, or SCR, to convert DEF to ammonia, which further breaks down into nitrogen oxide (NOx), a much cleaner nitrogen/water compound. To accomplish this, the SCR system uses an injector to introduce DEF into the exhaust stream of a diesel engine. At this point, a chemical reaction takes place, converting nitrogen, water, and trace amounts of carbon dioxide.

DEF storage temperature can go a long way in maintaining shelf life. When stored out of direct sunlight and in a cool place (between 23 and 68 degrees Fahrenheit), DEF is usually good for about two years before it needs to be used.

Get an even more in-depth background on DEF in our recent article, ‘What Does DEF Do?

How DEF System Contamination Happens

It can be harmful (and costly) to put anything other than diesel exhaust fluid into a DEF tank.

Sometimes, DEF winds up in the diesel fuel tank due to an honest accident. Although fuel nozzles won’t fit into a DEF tank, diesel fuel tank doors typically accommodate DEF nozzles with ease.

Another mistake that can lead to DEF system contamination is using DEF storage containers for other purposes and cross-contaminating. For example, DEF (a water-based fluid) does not mix well with oil. If a DEF jug is filled with oil, then later filled with DEF prior to adding it to the DEF tank, the oil residues can actually ruin the entire SCR system, causing damage that’s not covered under warranty.

The same issue occurs in reverse, when a storage container is first used for water-based DEF fuel, prior to using it to store oil. When contaminated oil is added to the engine, water droplets turn to steam under the pressure and heat of the engine, causing engine trouble and corrosion. Severe damage can occur in areas with soft metals, like the transmission.

Know When Your DEF is Contaminated

One of the first signs of DEF presence in engine areas is the Check Engine light turning on. Don’t ignore this, as damage may occur and your vehicle could shut down. In addition, a noted increase in the amount of DEF used, or malfunctions in the SCR system may be warning signs that your DEF has been contaminated.

To determine if the DEF in the SCR system is contaminated, take a look at the diesel exhaust fluid. Contaminated or old and possibly expired DEF will be noticeably cloudy, while pure, uncontaminated DEF will always be clear.

The best way to monitor DEF, however, is to use a digital refractometer to measure the concentration of urea in the fluid. Measurements should read between 32.5 and 37 percent to ensure clean DEF in your system.

How to Clean a Contaminated DEF Tank

If there’s any possibility that the DEF tank may have been filled with something other than diesel exhaust fluid, drain and flush the tank with deionized water or clean diesel exhaust fluid. If you’ve accidentally added DEF to the fuel tank, don’t operate the vehicle until you’re sure that only fuel is in the fuel tank. It’s easier to wait and drain the tank completely than it is to deal with the repairs that might be needed later.

Once your DEF tank is empty, warm water is all that’s needed to rinse it out. Once the tank is clean and reassembled, fill the reductant tank with fresh diesel exhaust fluid.

Gasoline in a DEF Tank

If gasoline gets in your vehicle’s diesel exhaust fluid tank, it could defile the DEF and damage the entire SCR system. As soon as you realize there’s gasoline in the DEF tank, you should drain the DEF tank and wash it out to remove any traces of fuel.

Water in a DEF Tank

Since DEF is mostly water that’s been deionized, you may wonder if you can put water in your DEF storage tank. The answer is an emphatic no. Deionized water and tap water are very different, since tap water contains minerals and ions. SCR systems are designed to use diesel exhaust fluid, not water. Water contamination in the SCR system could harm the SCR system and tap water will not bring about the catalytic conversion required to reduce NOx emissions.

Having water in your diesel fuel tank is an entirely different problem, although it’s just as serious. Read up on the importance of removing water from diesel fuel tanks, and how to do it.

How to Dispose of DEF Fluid

If you need to dispose of DEF that’s gone bad or is contaminated, do so ethically. Don’t pour DEF down a storm drain or dump it outside. Check with environmental protection laws in your community for the best disposal practices.

In the event of a DEF spill, contain the liquid with sand and then shovel the material into a container for disposal. If DEF is spilled on your vehicle, rinse it off with water.


Focusing on preventative efforts will always be easier and better for your vehicle than waiting to solve a problem after it occurs. As experts in the industry, we’re proud to offer a range of solutions to keep both engines and businesses running smoothly.

If you’re unsure where to begin, our fuel & lubricant services and equipment maintenance solutions are a great start. Or, reach out and let us know what we can do for you!


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