Diesel Exhaust Fluid Dosing Malfunction – What Could Be Wrong?

Heavy vehicles like trucks, buses, and trailers have diesel engines that use diesel to run. However, these vehicles can produce black smoke from their exhausts if the diesel's exhaust fluid dosing malfunctions. What causes this problem, and how can you solve it? We consulted with the diesel experts, that gave us the following information.

Your Diesel Exhaust Fluid Dosing can malfunction due to the following reasons:

  • Crystal Deposits
  • Low temperatures
  • Dirty fluid
  • Too much water in the DEF 
  • Adding other fluids

Keep reading to know why your Diesel Exhaust Fluid dosing is malfunctioning. Also, learn how the system works and the cost of fixing this system. Lastly, we tackle when you should refill the fluid and the pros and cons of the same.

Putting DEF to the car tank, Diesel Exhaust Fluid Dosing Malfunction - What Could Be Wrong?

Diesel Exhaust Fluid Dosing Problems?

The Diesel Exhaust Fluid [DEF] is sometimes known as AdBlue. It is the main component in a chemical process called Selective Catalytic Reduction [SCR]. This system works to reduce harmful emissions. During combustion, your engine produces NOx, forming smoke.

AdBlue DEF gas station

DEF, a mixture of 32.5% urea and 67.5% de-ionized water, is injected into the exhaust system. It turns harmful Nitrogen Oxide (NOx) into nitrogen and water. As a result, your vehicle produces less harmful emissions. 

However, problems can arise causing the DEF dosing to malfunction, including:

Crystal Deposits

The quality of DEF can be interfered with by adding too much fluid to the system. It can cause the DEF not to hydrolyze completely, crystallizing it to form crystal deposits inside the injector nozzle or exhaust. 

Too much DEF can happen if the DEF pump has problems or a DEF injector nozzle is partly open. Also, topping the DEF system with hard water can lead to the formation of crystal deposits. 

Low Temperatures

DEF can freeze in temperatures below 12 degrees Fahrenheit, preventing the engine from functioning optimally. When this happens, don't add additives to thaw the fluid. You could damage your SCR system or cause an imbalance in the fluid's concentration.

When creating the SCR systems, the OEM considered the expansion of tanks that holds the DEF. They included a heating system that turns DEF into liquid. The heating system thaws the DEF when you start your vehicle and when you continue operating your vehicle as usual.  

Dirty Fluid

DEF needs to be handled with utmost care to prevent it from being contaminated. Store DEF only in dedicated tanks. If you have to remove one product, and put in DEF, ensure you clean and rinse the tanks or containers with de-ionized or distilled water. 

When pouring the fluid into a tank, use dedicated containers and equipment such as funnels and hoses. For example, don't use the same hose pipe that poured another liquid to pour DEF. 

Mechanic opening the car engine

DEF can corrode anything that contains brass, aluminum, zinc, chrome, or nickel. Therefore, do not store it in containers that contain these metal elements. Any corrosion that takes place contaminates the exhaust fluid. 

Too Much Water In The DEF

As earlier mentioned, adding too much water to the fluid can cause it to crystalize. Also, nitrogen oxide will not be neutralized, causing more of this harmful gas to be emitted.

Other Fluids

Contaminating DEF from fluids such as diesel fuel or engine oil should be avoided. It can cause the SCR system to malfunction and the engine to shut down.

What To Do When Your Vehicle Runs Out Of DEF?

Check out this diesel exhaust fluid on Amazon.

Diesel exhaust fluid is crucial to keep your vehicle running efficiently. As earlier mentioned, DEF is a combination of water and urea. It gets injected into your exhaust, cleans the harmful nitrogen oxide, and emits it as clean air. Thus, you should ensure your tank has enough DEF at all times.  

A tank that runs dry may not run efficiently as it did before. As a result, it may be costly to repair any damages that arise. Therefore, refill your tank when you see a warning light on your dashboard indicating your DEF is low. 

If your tank does run dry, you could still revive your vehicle by adding more DEF. It should take a few minutes. 

This is a DEF header that you can find on Amazon.

A dry tank can lead to huge problems, one of them being the DEF header breaking. The header is the sensor that reads the temperature, level, and quality of the fluid. Replacing this part can be expensive. 

If, for whatever reason, your vehicle keeps running out of DEF, too much soot and debris could clog the exhaust system. Cleaning the exhaust system to remove the soot could take several days, sometimes weeks. It all depends on how much soot accumulated. Remember, as soot is piling up, some parts may be damaged.

How To Ensure Your Vehicle Has Enough DEF

AdBlue DEF cap photographed up close

The Environmental Protection Agency [EPA] has provided the DEF minimum level that your vehicle should have. You can find the fluid in hardware stores, auto-part stores, truck stops, and gas stations.

This means it's almost everywhere. Thus, if you run out of it, look around and see which shop or vendor sells the fluid.

And, if you have to buy and take some home or to your office, ensure you store it in a dry and cool place, away from direct sunlight. 

Furthermore, when refilling the liquid, do not confuse the tanks. Generally, the DEF tank is right next to the fuel tank. You can differentiate the two by the color of the caps. The diesel tank has a green cap, while the DEF tank has a blue cap.

Have a look at the fuel and DEF caps on Amazon.

Pros And Cons Of DEF

Knowing the advantages of using DEF in your vehicle and the disadvantages, if any, would be good. You can find them below:


Clean Air

As discussed above, the main advantage of DEF is that your vehicle emits clean air that does not pollute the environment.  

Better Fuel Efficiency

When the DEF converts the harmful NOx into harmless air, the performance of your engine improves. When your engine performs better, fuel consumption improves.

Reduced Wear And Tear

DEF improves the overall performance of your vehicle's engine, resulting in less wear and tear on engine parts. 

Less Maintenance

You will have less maintenance when your engine runs efficiently by ensuring the DEF levels are adequate. This, of course, means fewer costs. 


The only con that comes with DEF is the additional costs associated with installing the storage tanks. By 2010, the EPA required that all trucks install storage tanks for the exhaust fluid.  

How Long Does DEF Last?

A green Diesel cap and an AdBlue DEF cap

If you will be storing DEF to use later, it will degrade with time. But if you store it between 10 and 90 degrees, it can last one year. You could, however, extend its shelf life by storing it at around 75 degrees Fahrenheit. 

Generally, DEF tanks use one gallon of exhaust fluid for every 50 gallons of diesel fuel. One gallon should last between 300 and 500 miles. You can drive your vehicle for 800 miles, using 2.5 gallons.

Don't forget to refill your DEF tank when the warning light appears on your dashboard. You can prevent hefty repairs, as seen in the following paragraph.

Cost To Repair The SCR System

Replacing a DEF header would cost around $1,200. The catalytic converter can fail and can cost between $300 and $3,000 to repair, depending on the vehicle you drive. 

A DEF tank of a huge truck

To replace the DEF heater can cost around $800, including parts and labor. Labor costs to clean your DEF filter are about $1500. To replace a damaged filter costs between $2,000 and $5,000.

It's important to ensure your vehicle has enough DEF because the damage caused can be expensive to repair. There have been cases where repairs have reached up to $20,000.

In Closing

Putting DEF to the car tank

DEF helps your truck to emit clean air. The harmful nitrogen oxide is converted to less harmful emissions, resulting in cleaner air.

Problems with your DEF that could interfere with the exhaust system are mentioned here. For a vehicle to run efficiently, ensure it has enough DEF. A dry tank can cause damage to the exhaust system or engine parts.

The costs of repairing the exhaust system depend on many factors, the main one being the model of your vehicle. The other factor is dependent on the cost of the parts. 

You are welcome to read our previous posts below:

When And How To Add Def Fluid

Does Def Go Bad

Best Def For Ford F250

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