9 Common AdBlue System Problems & Solutions

Modern cars, diesel in particular, are getting cleaner and cleaner with each generation. According to numerous reports and environmental policies, their era is coming to an end as they face an electric extinction.

But until then, car manufacturers must push the limits to comply with emission standards and for the past 7 years, the best way to clean up diesel engine’s is by implementing an SCR (Selective Catalytic Reduction) exhaust system that features an AdBlue system.

This article will reveal all common AdBlue System problems and their solutions. We will turn our focus to Mercedes-Benz AdBlue problems, but the problems are generally very similar across all car brands.

What is AdBlue?

AdBlue is a brand name of a popular diesel exhaust fluid. It is a non-toxic, colorless solution of demineralized water and urea. Yes, Urea, the same substance that you find in urine.

A light spray of urea and demi water (AdBlue – diesel exhaust fluid) in the SCR catalyst reduces the amount of harmful mono-nitrogen oxides that eventually end up in our environment.

AdBlue systems are nothing new. The technology has been used in trucks and buses since 2005. Passenger diesel cars adopted the technology in 2016 with the EURO 6 emissions standard.

Here is a simplified display of how the SCR – selective catalytic reduction process goes.

  1. Exhaust fumes exit the engine.
  2. The fumes make their way through the Diesel Particulate Filter (DPF) where the majority of the dangerous soot particle get trapped
  3. Before the fumes make their way into the SCR catalyst an NOx sensor measures the engine out amounts of harmful mono-nitrous-oxides (NOx).
  4. Based on the NOx sensor reading, the AdBlue control unit determines the amount of fluid needed to clean up the NOx.
  5. As the fumes enter the SCR catalyst, an injector sprays the pre-determined amount of diesel exhaust fluid and which cleans up the NOx from the exhaust fumes.
  6. Another NOx sensor measures the amount of NOx that manage to escape the SCR catalyst. Together with nitrogen and water, the remainder of the NOx make their way out of the exhaust pipe.

Components and the process may vary based on the brand of the car, but it doesn’t vary greatly from the process we just described. Keep in mind that this is a very simplified version of the actual process, but it’s enough to understand the basic concept.

Does AdBlue reduce fuel consumption?

No, AdBlue use has no effect on fuel consumption, there is no logical connection which could explain how the use of SCR systems and AdBlue could contribute to a lower fuel consumption.

If you ever encounter any information that counters this fact, you should ignore it because it probably nothing but fancy marketing talk.

Why do Modern Diesel Cars Have AdBlue Systems?

Modern diesel cars use SCR system with AdBlue to lower the amount of harmful NOx (mono-nitrous-oxide) emissions. In order to comply with strict emission standards like the EURO 6, car manufacturers had to find a way to lower the emissions released by diesel engines.

Implementing an AdBlue system, similar to that of buses and trucks, significantly lowers the amount of emissions. This is the only reason why modern diesel cars have AdBlue systems.

Why is AdBlue not used in petrol vehicles?

Petrol cars in general produce much less harmful NOx emissions, which are a much more common bi-product of diesel engines. For that reason alone, the use of SCR systems and AdBlue is not needed.

Common AdBlue (SCR) System Problems

If you’re in a rush and you just want information fast, we got you.

These are the most common AdBlue or SCR system problems:

  • Diesel exhaust fluid (AdBlue) contamination
  • AdBlue injector failure
  • AdBlue pump failure
  • Electrical issues or control unit issues
  • SCR catalyst problems
  • NOx sensor failures
  • AdBlue level sensor failure
  • AdBlue heater/pre-heater failure
  • AdBlue tank leaks

Diesel exhaust fluid (AdBlue) contamination

One of the most common reasons for AdBlue problems is contamination. We’ve seen it all; drivers mixing up diesel fuel with AdBlue, drivers mixing different diesel exhaust fluids together, drivers filling up the AdBlue tank with water or just demineralized water, and the list goes on.

Mixing up diesel with AdBlue is the most common issue we see. Once you start up the car with diesel contaminated AdBlue fluid, you’re in for some trouble. Diesel fuel can cause major damage to the SCR catalyst, so if you happen to pour some diesel in your AdBlue tanks, it’s best to take action immediately, don’t turn on your car and drive.

Another common problem we see is saving money. The truth is, AdBlue is not that expensive, and it’s not like you have to fill it up every week. But we still see people buying cheap, unstandardized diesel exhaust fluids that only lead to issues down the road. There’s also people who think AdBlue is just fancy water, so they pour some water down into the AdBlue tank. Not a good idea.

Electrical issues or control unit issues

Control units of AdBlue systems are responsible for gathering data from the NOx sensors, calculating the amount of AdBlue needed at any given moments and making sure that everything goes to plan.

If the control unit fails, you’re most likely going to receive a warning light on your dashboard. In any case, there are going to be diagnostic trouble codes stored in your cars system, indicating AdBlue control unit issues.

Some of these issues can be repaired, especially with the know-how of independent shops, however, most official dealerships will have no problems just replacing the control unit with a new one (expect a repair bill higher than $1000).

AdBlue injector failure

The AdBlue system uses an injector to spray the calculate amount of diesel exhaust fluid into the SCR system to clean up those nasty emission’s. The injector itself is prone to both clogging and failure.

The first issue we mention is more common. Clogged up AdBlue injectors can lead to dashboard warning lights and even limp mode. If the injector’s electronic parts operate without a problem, the injector can be cleaned with ultrasonic baths, and you should be ready to go in no time and without a wallet braking cost.

If the injector fails completely, you’re looking at a $100 to a $500 repair bill, including labor (depends on the brand and the engine).

AdBlue pump failure

As you might have expected, every system with an injector must feature a pump that delivers the juice to the injector. The AdBlue system is no different, and just like very other pump, it’s prone to failure at some point. The reason for failure can either be in the electronics of the pump, or entirely mechanical.

There are independent shops that try and repair AdBlue pumps, but more commonly, owners just have to replace them with new ones, which can cost you up to $500 or more.

SCR catalyst problems & failure

This could be filed mainly as a consequence of AdBlue contamination or neglect in the system’s maintenance. The selective catalytic reduction system is unlikely to fail without a good reason behind it.

According to a 2021 research on SCR failure, an insufficient amount of ammonia (NH3), which reduces the conversion efficiency of NOx, leads to excess NOx emmisions and results in NH3-SCR failure. The most common reasons for this are insufficient urea injection (a key component of AdBlue is urea!), hydrothermal aging (time + heat = damage) of the catalyst and urea crystallization (caused by urea deposits in the catalyst).

NOx sensor failures

One of the most common SCR system related issues you’ll hear about are the dreaded NOx sensor failures. They are essential for the operation of the AdBlue/SCR system, but they are the most common reason for anger amongst the owners.

For instance, a failed NOx sensor on a Mercedes-Benz can prevent the car from running, in order to prevent the failure of other SCR components as well. Replacing the sensors isn’t cheap as well. OEM NOx sensors can cost up to $350, which is a lot for something that isn’t considered “essential” by the majority of owners.

AdBlue level sensor failure

To make sure you don’t run out of AdBlue, the majority of cars come with numerous alarms and digital gauges where you can track the amount of AdBlue.

Running out of AdBlue will usually trigger your car to enter the limp mode, limiting its performance to reduce the amount of emissions.

Unless you’re completely ignorant of all the warning signs that your car is flashing, you’re probably never going to run out of AdBlue. However, we did see cases of AdBlue level sensor malfunctions where people thought they have plenty of AdBlue, but in reality it was their level sensor telling them lies.

If you are the only one driving your car, try to remember the interval at which you typically top up the AdBlue fluid. If you think your sensor might be malfunctioning, you can top up the fluid prematurely and see how much of it was missing, then compare that to your sensor reading.

AdBlue heater/pre-heater failure

To prevent the diesel exhaust fluid/AdBlue from freezing, every SCR equipped car features a heating element inside the AdBlue tank. If it fails, your car will immediately store diagnostic fault codes or even flash warning lights.

Without the heating element, the AdBlue can freeze, expand and crack open the AdBlue tank, lines and pump, which results in enormous repair costs.

AdBlue tank and pipe leaks

Last but not least, a common AdBlue system problem is leaking. Due to time and exposure to the environmental elements, numerous pipes, connectors, and seals may start leaking. We’ve also witnessed leaks coming from tiny cracks on the AdBlue tank.

Taking care of any leaks should be your priority.

Common Mercedes AdBlue Problems

Here at Lifeonfour.co, we aim the majority of our efforts toward Mercedes engine problems and buyers’ guides, so we think it’s only fair we look at the AdBlue problems of this brand.

The AdBlue or SCR system of Mercedes-Benz is no different to the systems of other brands.

Common Mercedes AdBlue problems include:

  • AdBlue contamination
  • AdBlue injector failure
  • AdBlue pump failure
  • Electrical issues or control unit issues
  • Urea crystallization
  • NOx sensor failures
  • AdBlue level sensor failure
  • AdBlue heater element failure
  • AdBlue tank leaks

We described all of these issues in detail above, however, there are different AdBlue warning messages, which your Mercedes might pop up when there’s AdNlue related trouble.

These include:

  1. AdBlue Check Additive See Operator’s Manual
  2. Check AdBlue Remaining starts: X
  3. Check AdBlue No engine Start Possible in X miles/km.
  4. Engine Start Not Permitted (AdBlue Empty)
  5. AdBlue Range

Tho of those messages are especially interesting, the 2nd one and the 3rd one.

The 2nd one “Check AdBlue Remaining starts: X” indicates that there is something wrong with the AdBlue system and that the car will only allow you a certain amount of engine starts before it prevents you from starting.

The 3rd one shows you how many miles can you cover, before the car disables your ability to turn on the car and drive. Sounds crazy, right?

That’s just how it is. There is no joking around when it comes to emissions, or so it seems. There are ways to reset those Mercedes AdBlue warning messages, but we always recommend checking what part is at fault first. An experienced mechanic with Mercedes Star diagnostic tools should have no problems figuring out what part of the AdBlue system is causing the problem.

While it is true that diagnostic tools can get rid of those warning and that your Mercedes will run perfectly fun without the AdBlue system, it’s also true that sometimes a warning message can pop up just because you slightly overfilled the AdBlue tank.

In that case, it’s better to drain some AdBlue fluid, reset the message and have a fully functioning car. In many cases tho, owner’s get faced with expensive AdBlue repairs, so they decide to just erase those warning messages as they pop up, or simply delete the entire AdBlue system in their Mercedes-Benz completely.

Deleting the AdBlue system will not affect the engine, it’s going to run perfectly fine without it. However, you are going to be letting out much more harmful emissions, and you may even run into problems with the law.

Certain countries do not check the NOx emissions at the yearly technical vehicle inspections, others like Germany or Austria for instance do. There’s a big fine coming your way if the authorities find you driving around with a malfunctioning AdBlue system. Even if you’re a foreigner visiting their country, we’ve heard numerous stories of this happening. So keep this in mind!

Mercedes AdBlue Delete Problems

So you decided to avoid the potentially enormous AdBlue system repair costs, and you just want to get rid of the system entirely. Ok, but it’s not as simple as it might seem.

  1. AdBlue Delete DPF problems
    Even though you deleted the AdBlue system, you should keep in mind that your DPF system still needs to work. Many amateur programmers and shops, will delete your AdBlue system, but with it, they will also disengage the regeneration cycle of the DPF filter.

    Deleting the AdBlue system means that the ECU needs to be reprogrammed, and that’s not a job you want an amateur to deal with.

    This can turn into a real nightmare once your DPF clogs up and suffocates your engine.
  2. Consider the laws in your country.
    As we mentioned earlier, deleting the AdBlue system is a serious offense in many countries, Germany and Austria being one of them. Because this is so common, the police have the know-how to check if the AdBlue system has been removed or disengaged.
  3. Consider the environment.
    We get it, paying $1500 for an AdBlue repair is a nightmare, especially when you know that your car can run without it just as fine. This is the reason many owners choose the “AdBlue delete” procedure, which only costs around $200.

    The truth is, the AdBlue system really works when it comes to lowering harmful emissions, and if you can afford a car with an AdBlue system, you should be prepared to fix it when the issues arise. Some people do it just because they care about the environment they live in.

How to Avoid Common AdBlue Issues

The majority of AdBlue problems can be easily avoided. And yes, it is true that you can prevent the NOx sensors from randomly failing, but you can do many other things to avoid hefty AdBlue repairs.

Never mix up or contaminate the AdBlue fluid

The AdBlue filler cap and the diesel fuel tank cap are usually very close together (but they look much different). However, many people still manage to mix up the two and pour some diesel down that AdBlue drain. Mixing AdBlue with anything else than AdBlue leads to major SCR system damage and big repair costs.

The same goes for mixing AdBlue fluids. Just don’t do it. Stick to one brand and make sure it’s a certified (ISO 22241 standard), standardized diesel exhaust fluid, compatible with your vehicle. Avoid using cheap AdBlue alternative with questionable quality.

Store AdBlue in appropriate conditions

If you happen to have some AdBlue in stock at home (which is a great idea), make sure you store it in a cool, dry locations where the temperature never goes below -10 degrees Celsius.

Once AdBlue fluid freezes up, it changes the quality of the fluid and it’s no good (others may disagree, but we like to stay safe). You should also avoid keeping AdBlue in direct sun light and in spaces where the temperatures rise above 30 degrees Celsius.

If you keep some AdBlue in stock, make sure you use it withing a year due to time-related degradation.

Never run out of AdBlue

To avoid getting stranded or put into limp mode, you should always keep an eye out on the AdBlue levels. Below is an image of the AdBlue level indicator from a 2017 W213 Mercedes E-Class.

The red arrow on the right indicates the reserve tank level, while the blue line at the top indicates the actual AdBlue level.

Avoid short journeys with a diesel car

The SCR system only works once the car fully warms up and the exhaust temperatures reach high levels. If you only drive your diesel on short journeys, the urea in the AdBlue fluid can crystallize and clog up the pipes and the injector. That’s just one more reason why diesels aren’t cars for short commutes.


For the majority of second hand buyers, AdBlue issues are just another thing to worry about. Not only are you responsible for keeping your engine and transmission in good condition, now there’s also an entire system connected to your exhaust, with several components that can easily fail and lead to high repair costs.

If you’re buying a used diesel car with an SCR system, make sure it works as it should or, better yet, make sure it wasn’t deleted, and you’re at a risk of breaking the law.

Frequently Asked Questions

Does adding AdBlue to diesel cause major problems?

Accidentally adding AdBlue solution to the diesel fuel tank can lead to catastrophic engine damage. AdBlue is a solution of demineralized water and urea and it has no place in the actual diesel engine. It is not a diesel ful additive.
Does AdBlue in a car cause problems?

No, AdBlue in itself does not cause problems to the car. It doesn’t impact its longevity or performance. It can however increase the costs of maintenance as many AdBlue or SCR system components tend to be expensive to replace when they malfunction or fail.
Does AdBlue cause DPF problems?

No. There is no evidence to support the claim that AdBlue causes DPF problems. Both system work hand in hand to lower the amounts of harmful environmental emissions.

Deleting the AdBlue systems from a diesel car, however, can lead to DPF problems.

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