What is AdBlue?
AdBlue is designed to make your diesel car more efficient and was introduced to meet Euro Six emission regulations. So, what does it consist of?
The Cazoo editorial team
Published on 20 November 2025 | 1 min read
You might think that diesel cars are dead, right? Well, diesels are still a very popular choice for those who cover lots of miles and need a vehicle that is going to be economical and reliable.
If you drive a modern diesel, you might have noticed a blue-coloured cap next to the fuel door, which is labelled ‘AdBlue’.
There are thousands of diesel cars on Britain's roads that now use AdBlue, with lots available through Cazoo.
However, if you’re unsure what AdBlue is, we’ve compiled a guide to help you understand a bit more on what it actually does to your diesel vehicle.
What is AdBlue?
So, what is AdBlue? AdBlue is a diesel exhaust fluid that was introduced in the wake of the Euro Six emission regulations.
It’s a non-toxic, non-flammable, odourless and biodegradable fluid which works in conjunction with a treatment system that is called the ‘Selective Catalytic Reduction’ (SCR), which breaks down any harmful nitrous oxide (NOx) emissions from exiting a diesel vehicle’s exhausts.
AdBlue is stored in a separate tank of a diesel vehicle and is made up of an organic compound called ‘urea’ and water solution. The urea contains ammonia, which reacts to NOx and prevents it from being released into the atmosphere, which helps improve air quality, makes your diesel vehicle more efficient and produces less CO2 emissions.
When was AdBlue introduced?
AdBlue has been around since the mid-2000s; however, it was in September 2015, in response to the upcoming Euro Six emissions regulations, when AdBlue became increasingly popular among car makers.
Most diesel vehicles registered after this date will now feature an AdBlue system in a bid to help cut down on stricter emission targets and improve air quality.
How does it work?
It’s relatively simple how AdBlue works. When a diesel car is running, the AdBlue will be injected into certain sections of the vehicle’s exhaust system which then causes a chemical reaction within the unit to break down and remove any harmful nitrogen-oxide particulates that will exit the exhaust and convert them into water and nitrogen.
Just like with fuel, AdBlue will need to be topped up over a period of time, with most cars that feature the system having a warning light on the dashboard to alert you when the fluid is getting low.
Some cars, for example, will even give you an estimated range of driving before the AdBlue will run out. The AdBlue will last longer or shorter depending on how you drive the vehicle, how big the tank is, how much fuel the vehicle is consuming and your driving style.
Filling up with AdBlue is a straightforward process. Firstly, you’ll need to buy an AdBlue solution, which you can get from any motoring store or even the vehicle manufacturer. Turn off the vehicle’s engine and park it on a flat surface. Then, open the AdBlue cover located next to the fuel cap and begin to insert the nozzle of the AdBlue container into the tank and start filling. Once it reaches the maximum level, stop pouring and securely fasten the AdBlue cap back up and you’re good to go.
What are the positives?
There are lots of upsides to AdBlue, with the biggest factor being it reduces a diesel vehicle’s NOx emissions, improves air quality and lowers CO2 output.
Also, it benefits diesel drivers driving into cities with London’s ‘Ultra Low Emission Zone’, with every diesel vehicle registered after 2015 compliant with the scheme, which makes it cheaper for those who need to drive into the city centre regularly.
AdBlue and the SCR system can also help protect your engine’s components from any premature damage, as it helps reduce high levels of NOx, which can contribute to more engine wear.
Plus, AdBlue is safe to handle and isn’t flammable, toxic and is considered a ‘low risk’ when it’s being transported from manufacturing to retail.
What are the negatives?
As AdBlue will have to be topped up once it’s empty, that means it increases your additional running costs to your vehicle. Also, the SCR system will need to be regularly checked to avoid any issues or failures.
Other issues include the fluid crystalising in colder temperatures which can lead to injectors clogging and the engine running rough. Plus, AdBlue is a corrosive substance which can cause damage to certain engine parts that contain copper if not handled correctly. And, AdBlue freezes in temperatures from -11 °C or lower, meaning your diesel vehicle will not start in extremely cold conditions.
