What Is AdBlue and How Does It Work?
Owning a diesel · Published
AdBlue is a urea solution that lets modern diesels clean up their exhaust. Here is what it is, how the system works and what happens when it runs low.
Reviewed by the Bolt Remaps technician team: Dean Rogers, Matthew Jones, Irwin Luis and Arthur Kupce. Last reviewed .
In short
- AdBlue is 32.5% urea in demineralised water, made to the ISO 22241 standard.
- It is sprayed into the exhaust so a catalyst can turn harmful NOx into nitrogen and water.
- Most Euro 6 diesels need it. Use per mile varies by vehicle, so check the handbook.
- If it runs out, a warning and countdown start. In the end the engine may refuse to restart.
What AdBlue actually is
AdBlue is the trade name for a diesel exhaust fluid. It is a clear liquid made of 32.5% high purity urea and 67.5% demineralised water.
The mix is set by the ISO 22241 standard. That standard covers purity and concentration. It exists because the fluid goes into a precise emissions system, and impurities can damage it.
AdBlue is not a fuel and it is not an additive for the diesel tank. It is stored in its own tank and used only by the exhaust system. It is non-flammable and not classed as toxic, but it can irritate skin and eyes, and it is corrosive to some metals.
How SCR works
SCR stands for selective catalytic reduction. It is the system that uses AdBlue. In simple terms, it works like this.
Fluid is stored
AdBlue sits in a dedicated tank, usually with its own filler next to the diesel filler or in the boot or under the bonnet.
A pump and injector dose it
A pump builds pressure and a dosing injector sprays a small, measured amount into the hot exhaust stream upstream of the SCR catalyst. Some systems use an air-assisted design, and the details vary by make.
Heat turns urea into ammonia
In the hot exhaust, the urea breaks down into ammonia. This needs the exhaust to be warm enough, which is one reason short cold journeys can be hard on the system.
The catalyst does the cleaning
Inside the SCR catalyst, ammonia reacts with nitrogen oxides (NOx). The result is nitrogen and water vapour, both of which are already in the air we breathe.
Sensors check the result
NOx sensors measure the exhaust before and after the catalyst. The engine computer uses them to adjust dosing and to spot faults.
Why Euro 6 diesels need it
Diesel engines run lean and hot. That is good for economy, but it makes a lot of NOx. NOx is a harmful gas linked to poor air quality, especially in towns and cities.
Euro 6 emissions rules set tight limits on NOx from diesel cars and vans. Many manufacturers meet those limits with SCR and AdBlue. Others use different methods, such as a NOx storage catalyst, so not every Euro 6 diesel uses AdBlue. If you are unsure, look for a blue filler cap or check the handbook.
Because the limits are tight, the car monitors the system closely. If it cannot confirm that dosing is working, it logs a fault and warns you. That is why a small problem can show up as a warning light long before you notice any change in how the vehicle drives.
How much AdBlue does a vehicle use?
It varies by vehicle. Engine size, driving style, load, towing and how hard the engine works all make a difference. Manufacturers give their own figures, so check the handbook for your model.
As a rough idea only, many cars use a small amount per thousand miles, and heavier vans can use more. Do not treat that as a rule. Tank sizes also differ a lot between models, so the distance you get from a full tank can be quite different from a similar vehicle.
Most vehicles show a range or a warning when the level gets low. Many give the first warning with a good number of miles still left, but the exact point depends on the make. Top up at the first warning rather than waiting.
What happens if AdBlue runs out
The system cannot clean the exhaust without fluid. So the car warns you first, usually with a dash message and a light. Warnings often get more urgent as the level drops.
If you ignore them, the vehicle will normally start a countdown. This shows a distance or number of starts left. When it reaches zero, many vehicles will not restart after the engine is switched off. Some limit power instead. What happens depends on the make and model.
Topping up usually clears a warning that was caused by low fluid alone. If the warning stays after a top-up, there may be a fault in the system, such as a blocked injector, a heater problem or a sensor fault. That needs a proper diagnosis with the right equipment.
Running very low can also let air into the pump on some systems, which may need a longer drive or a reset to clear. Do not keep running the tank dry on purpose.
When to call us
If your AdBlue warning stays on after a top-up, or the countdown is running, we can help. Bolt Remaps is fully mobile, so we come to the vehicle. We are IMI affiliated and have been fixing AdBlue faults since 2019.
The price is fixed and agreed before any work starts, from £249. If we do not fix the agreed fault, you do not pay. Repairs come with a written warranty. Call 0203 038 8555, 6am to 9pm, 7 days a week, including bank holidays.
Common questions
Is AdBlue the same as diesel additive?
No. AdBlue is a separate fluid with its own tank. It never goes in the diesel tank. It is used only by the exhaust system.
Does every diesel car use AdBlue?
No. Many newer diesels do, especially Euro 6 models and vans, but older cars and some newer ones use other emissions methods. A blue filler cap or the handbook will tell you.
What does ISO 22241 mean?
It is the standard that sets the purity and 32.5% urea concentration of AdBlue. Fluid made to this standard is the right grade for your vehicle.
How often will I need to top up?
It varies by vehicle and how you drive. Some owners top up once or twice a year, others more often. Your handbook gives guidance for your model.
Can AdBlue go off?
Yes, it can degrade if it is stored too warm, in sunlight or open to the air. Use fresh fluid from a sealed container and check the date on the label.
Will the car start if the tank is empty?
At first, usually yes, but you will get warnings and a countdown. Once the countdown ends, many vehicles will not restart. Top up before it gets that far.