What is AdBlue and what faults does it cause?

I remember the first time I had to fill the AdBlue tank perfectly. Not even two minutes had passed and I already had my jacket stained with that liquid that, although it looks harmless, can ruin more than the car’s anti-pollution system. Since then, I admit I look at old-school petrol cars with a certain nostalgia.

And the thing is, although AdBlue plays a key role in reducing emissions, it can also become a source of breakdowns, electronic errors and more than one headache. In this article we cover everything you need to know: what it is, when it was invented, which cars use it, what faults it can cause and how to avoid them.

What is AdBlue and what is it for?

AdBlue is a liquid made up of 32.5% synthetic urea and 67.5% deionised water. Its job is to reduce polluting emissions of nitrogen oxides (NOx) in diesel engines through a chemical process known as selective catalytic reduction (SCR).

When the vehicle is running, it is injected into the exhaust system. There it reacts with the hot gases and turns the NOx into nitrogen and water vapour, two components that are harmless to the environment.

The role of the SCR system in reducing emissions

This system is mandatory on many diesel models that meet the Euro 6 standard, because without it they could not pass current emissions checks. It is an effective and relatively inexpensive solution to reduce the environmental impact of diesel, but it is not free of drawbacks.

When was AdBlue invented and who developed it?

Although many people think AdBlue is a recent technology, the truth is that it began to be developed in the late 1990s in Germany. It was driven by the German Association of the Automotive Industry (VDA), which registered the “AdBlue” trademark and standardised the product for use in heavy vehicles.

Evolution with the Euro emissions standards

From 2006, the use of the SCR system and AdBlue became mandatory on trucks to meet the Euro IV standard. As emissions requirements became stricter, its use also spread to diesel vans and passenger cars, especially from the Euro 6 standard, in force since 2014.

Main problems in diesel cars

Although the AdBlue system helps reduce pollution, it can also be a source of breakdowns that are expensive and frustrating for drivers.

Frequent system faults

Some of the most common problems associated with AdBlue are:

• Crystallisation of the liquid, which blocks injectors and pipes.
• Failure of the SCR system pump, which prevents proper injection.
• Faulty sensors, which can throw errors even if the system is working.
• AdBlue freezing in cold climates, affecting start-up.

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What happens if a car runs out of AdBlue?

If AdBlue runs out, many vehicles will not start until it is refilled. The system is designed that way by regulation; it is not a fault, but an environmental safety measure.

Brands and models most affected

According to sources such as Grupo Royb and Diariomotor, some brands have had more AdBlue issues, especially:

• Peugeot/Citroën (PSA)
• Renault
• Mercedes-Benz
• Volkswagen and Audi
• BMW

This is due both to the configuration of the system and to its sensitivity to impurities, poor maintenance or weather conditions.

Practical solutions for AdBlue faults

Maintenance and prevention

To avoid AdBlue problems, follow these tips:

• Always use homologated AdBlue (ISO 22241).
• Check the tank regularly and refill before it runs out.
• Avoid leaving the vehicle parked for a long time with a full tank, because the liquid degrades.
• Clean up any spills if AdBlue is poured, because it can corrode metals and plastics.

Alternatives and recommendations

Although there are solutions such as disabling the SCR system or removing AdBlue use with modified ECUs, this is illegal and punishable in Spain. The best option is to keep the system working correctly and to be informed before you buy.

Which cars use it?

Most diesel passenger cars built from 2015 onwards include it.

Tips before you buy

• Check the maintenance history of the system
• Verify that there are no active electronic errors
• Ask about any past faults related to AdBlue
• Make sure the vehicle meets Euro 6 without modifications