5 Common Cummins DEF System Problems (Preventive Tips)

Cummins engines are popular for their power, reliability, and efficiency. If you are a truck enthusiast, you are bound to have come across the Cummins name at least once. Cummins fits a majority of its engines with Diesel Exhaust Fluid (DEF) systems with the main goal of reducing harmful emissions. But, this system is not without its issues. Today, we bring you the most common Cummins DEF system problems.

Contamination, crystallization, freezing due to cold temperatures, Urea dilution, and reduced fuel efficiency are the main DEF-related issues you should keep an eye out for when dealing with Cummins powerplants. Make sure to go through each of these carefully when making a new purchase.

Diesel Exhaust Fluid

Cummins DEF

“But, what exactly is a Cummins DEF system?” You may ask. We’ll answer that question first before diving head-first into the common problems you may come across.

The main responsibility of a DEF system is limiting the number of harmful emissions created by a diesel engine. Cummins is ahead of the curve from the rest of the competition in this aspect, as they have their own line of DEF fluid.

Cummins uses coal, natural gas, and petroleum products in the production of their DEF fluid. Similar to other diesel exhaust fluids out on the market, the one made by Cummins also consists of a mixture of Urea and deionized water in a 32.5% ratio.

There are several advantages of Cummins diesel exhaust fluid. First and foremost, it does not require any special handling. This allows Cummins owners with different levels of expertise to handle the fluid safely and securely.

Another great quality of Cummins DEF fluid is its non-toxic nature. Furthermore, this fluid is non-flammable, non-hazardous, and does not pollute the environment.

If you are the proud owner of a vehicle with a Cummins engine at heart, it is hard to recommend any other DEF fluid than this one.

Most Common Cummins DEF System Problems

Cummins DEF System

Now that you know all about Cummins and its DEF system, let us dive into the most common issues associated with it.

Some of the most common Diesel Exhaust Fluid-system related issues faced by Cummins engine owners include,

  • DEF contamination
  • Crystallization
  • Freezing DEF fluid
  • Fluid dilution
  • Poor gas mileage

Let us go through these problems one by one and see the causes behind them.

DEF Contamination

Contamination of the Diesel Exhaust Fluid is one of the most common issues with Cummins powerplants. DEF contamination can happen either during storage or when it is inside the vehicle.

You see, Diesel Exhaust Fluid is very sensitive to external elements. As a result, even the tiniest speck of dust can contaminate and reduce the efficiency of this fluid.

On most occasions, DEF contamination happens when the owner goes to refill it. Additionally, this can happen once you are merely checking the fluid levels as well.

Diesel Exhaust Fluid contamination can cause severe damage to the system itself, with the DEF pump taking the brunt of the impact.

There are several precautionary measures you can take to limit DEF contamination. We’ll take a look at these measures later on.


Another common issue with not just the Cummins but all DEF systems is crystallization. This happens when the water composition of the DEF fluid evaporates.

There are several things that can cause DEF crystallization to happen. The first and most common way is when owners top up the DEF reservoir with water instead of Diesel Exhaust Fluid. We do not recommend doing this as it can cause severe damage to the DEF system as well as increase harmful emissions.

Overdosing is another common cause of DEF crystallization. Overdosing refers to the act of adding excessive amounts of DEF into the system. This causes small crystals of DEF to form in the exhaust system and on the injector nozzles due to issues with hydrolyzation.

Clogged DEF lines, DEF pump issues as well as faulty DEF injector nozzle operation can all lead to DEF overdosing.

Freezing DEF Fluid

As you might have already guessed, the Diesel Exhaust Fluid should be in the optimal condition for it to work as intended. Freezing is another common problem Cummins engines face in this aspect.

The freezing temperature of DEF fluid lies at around 12ºF (-11 degrees Celsius). Although this is not a major issue when it happens inside a storage container, it is critical to prevent this from happening inside the vehicle itself.

If the Diesel Exhaust Fluid freezes inside the vehicle, it can negatively impact the engine operation. Oftentimes, this leads to thousands of dollars being spent on repair bills.

Also, we found it important to warn you against using additives to thaw the frozen DEF fluid. Doing so will only escalate the situation.

Fluid Dilution

Similar to what we mentioned earlier, DEF fluid is mainly a mixture of Urea and water. DEF dilution happens when the Urea concentration of the liquid is too low.

The NOx neutralization properties of DEF fluid directly tie into its Urea concentration. As a result, Diesel Exhaust Fluid with too little Urea is not as effective in removing harmful gases from the exhaust system.

The incorrect practice of topping up the DEF system with too much water is the main cause of Urea dilution. In addition to limiting the system’s effectiveness, DEF dilution can also lead to crystallization as well.

Poor Gas Mileage

In addition to helping with emissions, the DEF system impacts your Cummins engine’s fuel efficiency as well.

Some owners suggest that the DEF system reduces how far your vehicle can travel on a full tank of gas. If you are one of these owners, the best route to go down is deleting the system in its entirety.

Getting rid of the DEF system may improve the fuel efficiency of your vehicle by as much as 30%. However, keep in mind that the increase in gas mileage comes at the cost of engine emissions. Not only that, but the impact of this modification largely depends on the size of your vehicle as well.

How To Handle DEF Fluid?

Thought we were done? In that case, you are sorely mistaken.

The troubles Cummins owners face with the DEF system don’t end there. It is important to be well informed on how to handle this liquid with care as well.

If your Cummins comes with a DEF system, there is a high chance of you having some of this fluid stored at your home. This is where you have to be careful, as DEF fluid can go bad at a moment’s notice.

Ideally, you should store DEF fluid in a way that it doesn’t come into contact with direct sunlight. Doing so will break down the internal components of the DEF fluid, rendering it useless in the process. Instead, store the DEF fluid indoors where it doesn’t get too hot.

You have to pay special attention to the type of container being used as well. Use stainless steel or plastic bottles, and make sure that external elements like water or air don’t come into contact with it.

Preventive Maintenance Tips To Make Cummins DEF System Last Longer

Now, you have a good understanding of DEF-related issues Cummins owners might come across during their ownership. With that, it is time for us to explore some preventive actions that can limit these issues in the first place.

Monitor the urea concentration

Urea dilution is a major issue with DEF systems, and it is important to take the initiative to limit it as much as possible. Under ideal conditions, there should be between 31.8% – 33.2% Urea in Diesel Exhaust Fluid.

Cummins owners can monitor the Urea concentration using a device called a handheld DEF refractometer. You can submit the readings and samples from this device for various Urea content analysis tests as well.

Keep an eye out for contamination

Diesel Exhaust Fluid contamination is another issue that every Cummins owner should look out for. The easiest way to spot contamination is by simply looking at the fluid itself. If it appears colored or cloudy instead of being clear, your DEF fluid has been contaminated.

Other than the DEF fluid, you can inspect the storage containers and the DEF filter for signs of contamination as well.

However, dirt and debris are not the only ones that contaminate DEF fluid. Diesel, engine oil, hydraulic fluid, and antifreeze are all among the most common culprits that contaminate Diesel Exhaust Fluid.

Depending on the type of contaminant fluid, you can use different identification methods. For example, a smell test is enough to identify diesel contamination, while a sample test should be performed in order to check for hydraulic fluid.

No matter the cause, you should immediately drain the DEF fluid and replace it if any contaminants are present. By following these precautionary measures, you can extend the life of both the DEF system as well as the Cummins powerplant.

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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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