How To Tell If DEF Fluid Is Bad?

If you are a fan of diesel engines, you have probably heard of Diesel Exhaust Fluid. This is a special liquid that reduces harmful emissions from diesel power plants. But how do you tell when this fluid has gone bad?

Driving around with bad fluid not only damages your vehicle but harms the environment as well. After going through this article, you’ll understand how to tell when the diesel exhaust fluid has gone bad. Not only that, you’ll learn some ways to prevent it from happening in the first place.

In short, fluid discoloration, an illuminated DEF warning light, and poor engine performance are some symptoms that indicate that your vehicle has bad DEF fluid. Every diesel engine owner should keep an eye out for these common symptoms.

What Is Diesel Exhaust Fluid (DEF)?


It is important for you to have a good understanding of what Diesel Exhaust Fluid is before learning how to tell when it has gone bad. As we mentioned earlier, the main responsibility of DEF fluid is to reduce the harmful emissions from the engine.

Curious about what this fluid contains? Don’t sweat it; we’ve got you covered! It is a mixture of two components – synthetically manufactured urea and de-mineralized water. The exact composition stands at 32.5%

Vehicles with Selective Catalytic Reduction (SCR) technology are the ones that use DEF fluid. These vehicles have a separate tank to store the DEF fluid. When the engine runs, a small amount of DEF fluid gets sprayed into the exhaust pipe.

Once the liquid contacts the hot exhaust gases, the carbon dioxide and ammonia inside it breaks down. There, the ammonia gas reacts with the toxic nitrogen oxides in the exhaust gases, breaking it down to water vapor and nitrogen gas. These harmful bi-products then escape through the vehicle’s tailpipe.

However, not all diesel-powered vehicles come with DEF systems. This system is only found on vehicles that are mandated to have it by the Environmental Protection Agency (EPA). You can easily see whether your vehicle has a DEF system or not by going through its owner’s manual.

Cons of DEF Fluid

From what we have discussed so far, it is safe for you to assume that DEF fluid doesn’t have any drawbacks. But that is not the case. If you own a diesel truck with a DEF system, there are several cons that you should know about.

The First con of a DEF system is the additional weight they add to the vehicle. Most diesel vehicles are already heavy, so imagine the additional strain the DEF system and its fluid tank put on your vehicle.

Next up is the cost. Typically, vehicles equipped with DEF systems cost more upfront than ones that don’t come with one. This is something to keep in mind if you are a budget-conscious buyer.

But, we must also add that you’ll be saving back some of that initial expenditure through fuel savings. Also, the environmentalist will sleep soundly knowing that your diesel truck is emitting minimum harmful exhaust fumes to the environment.

You might want to consider setting aside some extra space to store an additional gallon or so of DEF fluid. Some people may consider this as a disadvantage, but you never know when you might need it!

Most Common Bad DEF Fluid Symptoms

Now that you know the importance of Diesel Exhaust Fluid, let us move on and discuss how you can identify whether it has gone bad. Expired DEF increases the toxicity of exhaust smoke while also affecting vehicle performance.

A few common symptoms of bad Diesel Exhaust Fluid are,

  • Discoloration
  • DEF Warning Light
  • Performance Issues


All it takes is one look at the DEF fluid to tell whether it has gone bad. Under ideal conditions, this liquid should have a clear shade. If it is cloudy or discolored, the fluid has most probably expired.

Breaking down of molecules is the main reason why Diesel Exhaust Fluid discolors. Additionally, contamination from other materials may result in discolored DEF fluid as well.

DEF Warning Light

Similar to the other essential systems on your vehicle, the DEF fluid has its own dedicated warning light on the dashboard. However, expired fluid is not the only reason why this warning symbol turns on.

If the DEF warning light stays constantly illuminated, your diesel vehicle is running low on DEF fluid. This is merely a cautionary light. Once you top up the fluid, the warning light will go away on its own.

On the other hand, a flashing DEF lamp indicates that the fluid level is below a critical level. This requires immediate action. On more severe occasions, the DEF warning light may turn on alongside the Red Stop symbol. If you don’t act fast, this carries the risk of permanent engine damage.

Performance Issues

As we mentioned earlier, bad or expired Diesel Exhaust Fluid severely affects your engine’s performance characteristics. You’ll experience everything from startup issues to a lack of acceleration and excessive smoke coming out of the tailpipe.

What To Do If DEF Fluid Has Gone Bad?

Let’s imagine for a second that the DEF fluid of your vehicle has gone bad. What do you do now?

Well, let us get the obvious out of the way first. You cannot reuse DEF fluid. Once it goes bad, you must replace it.

It is also important to be careful when disposing of Diesel Exhaust Fluid. Improper disposal of this fluid can cause health issues and damage the environment. So, we recommend leaving DEF fluid disposal to the professionals.

Preventive Maintenance Tips To Make Your DEF Fluid Last Longer

There are several preventive measures you can take to make your DEF fluid last longer. Now, we will take a look at some of the most useful ones.

Keep An Eye On The Temperature

Diesel Exhaust Fluid is sensitive to external temperatures. This is mainly due to its high water concentration. As a result, The DEF fluid may freeze or gel in colder climates below -11ºC.

Frozen DEF fluid can bring your engine down to its knees. To prevent this from happening, make sure to park your vehicle in an area with adequate heating. It is also important to store your excess DEF fluid under such conditions as well.

Store The Fluid Properly

Like gasoline, It is important to be vigilant when storing DEF fluid for future use as well. You should be careful and handle the product with care in order to prevent dirt and metal contamination.

Make sure only to store the DEF fluid in manufacturer-approved containers. These are typically made using polypropylene and stainless steel. However, reusing disposable DEF containers may lead to contamination.

Additionally, you must ensure the cleanliness of the DEF dispensing nozzles. Dirty nozzles are one of the main causes that lead to DEF fluid contamination.

Also, remember to properly label the DEF fluid containers. This will prevent confusion when the time comes to fill up your vehicle.

Similar to the other fluids you use in vehicles, DEF fluid also has a limited shelf life. If stored at ambient temperatures inside industry-standard containers, DEF fluid can safely last up to two years. However, exposure to excessive heat may cut down the shelf life in half.

Using contaminated DEF fluid in your vehicle can lead to a whole host of issues. DEF pump damage is one that immediately comes to mind.

Refill The DEF Fluid According To The Schedule

The frequency at which you should refill the DEF fluid varies depending on the vehicle you drive. Not only that, but the fuel efficiency of the diesel engine has an effect on this factor too.

On average, a modern light-duty truck can travel between 8,000 to 10,000 miles on 10 gallons of Diesel Exhaust Fluid. In earlier times, this figure stood at roughly 2-3 gallons of DEF fluid per 800 miles.

As you might have already guessed, the DEF consumption of medium and heavy-duty trucks is more than that. According to Cummins – one of the leading heavy-duty diesel engine manufacturers in the world, these vehicles consume around 2% DEF fluid when compared with the overall diesel consumption.

Other than the size of the engine, things like towing heavy loads also affect your overall vehicle’s DEF fluid consumption.

Most trucks come with gauges that tell you exactly how much DEF fluid remains in the tank. But keep in mind that this indicator also varies from one manufacturer to the next.

For example, Ford trucks have a mere low DEF light, while GM outfits their vehicles with a digital DEF level readout. RAM trucks come with an old-school gauge for that exact function.

If you own a diesel truck equipped with a Diesel Exhaust Fluid system, you should be proud of the way you are helping to save the environment. Just remember to keep an eye on the system because issues with this sensitive system can have a significant effect on your wallet.

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Mahir Ahmed


I'm the guy responsible for ensuring honest, informative, accurate and helpful guide to the reader.

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