AdBlue: Why does my car need it?

AdBlue: Why does my car need it?


If you’ve owned or driven a diesel car since 2015, or an HGV since 2009, you may have heard of the term, but do you know why it’s added to a diesel vehicle, and what it does? 

In this blog, we’ll talk all about AdBlue, including what it's made from, what happens if you don’t top up the tank and more. 

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What is AdBlue? 

Termed a Diesel Exhaust Fluid or DEF, AdBlue is an additive that some diesel engines use to reduce harmful emissions. It is stored in a separate tank, similar to the fuel tank, and requires filling, either by the driver or as part of a maintenance service.

Why is AdBlue needed? 

The Euro emissions regulations are updated every few years to improve the air quality around Europe. In 2015 the newest regulation: Euro 6, came into effect. 

This new set of regulations came with more stringent regulations on the level of harmful emissions diesel-engined cars are allowed to emit, in particular Nitrogen Oxide or Nox. 

AdBlue reduces the level of these pollutants by acting as a catalyst inside the SCR (Selective Catalytic Reduction) system, to ensure as much Nox is burned off, and transformed into harmless water and oxygen. 

What is AdBlue made of? 

AdBlue is a liquid that consists predominantly of water and urea, in roughly a 70/30 mix. There are also other additives and chemicals added to stabilise and colour the fluid. 

Is Adblue made of pigs' urine? 

Whilst urea is present in urine from all animals, humans and pigs included, there are no animal by-products used in the making of AdBlue. 

The urea added to make the product is entirely synthetic, the main reason for this is that the SCR system is extremely sensitive to impurities. The extra compounds found in urine would cause a multitude of issues with your car's exhaust system. 

What is AdBlue used for? 

The SCR system in diesel cars was introduced to reduce the emission of Nox pollutants. It does this by combining an exhaust fluid, with the stream of hot exhaust gases inside a catalytic converter.  

AdBlue is one of the most common DEFs (Diesel Exhaust Fluids) and is fed from a separate tank, usually stored near the diesel tank, to be injected into the exhaust.  

When operating properly, an SCR system (Selective Catalytic Reduction) combined with an EGR (Exhaust Gas Recirculation) system can reduce harmful Nox emissions by up to 90% 

What happens if you run out of AdBlue? 

Cars that require the addition of AdBlue do so because their SCR systems cannot function without it. Any diesel car made after 2015 is allowed to exhaust a certain level of Nox pollution.  

Many cars, therefore, will go either into a reduced power mode also known as a limp home mode or refuse to start together, if no AdBlue is present. 

That is why it’s a good idea to keep a small bottle in your car along with other essentials, just in case you’re caught short. 

What happens if you put diesel in an AdBlue tank? 

Diesel Exhaust Fuels are specifically designed to be used in a separate system from regular diesel fuel. The AdBlue should not enter the engine under any circumstances.  

AdBlue tanks filler caps are often located in the fuel filler door, the filler hole is too small to allow diesel to be put in, but an AdBlue bottle could be put into the diesel tank. 

If you accidentally put AdBlue in your diesel tank, call your emergency roadside assistance as soon as possible, and don’t drive the car more than absolutely necessary.  

A small amount of AdBlue could probably be diluted into the fuel in order to cause no issue at all, but it’s best to get a professional opinion in your particular situation.  

Related Reading: How To Know If DPF Is Blocked 

How often do you need to fill up AdBlue? 

For the average car completing 9-10,000 miles a year, you should only need to buy and refill your AdBlue once per year. 

This is because although different cars use different amounts of AdBlue, a typical tank will hold 15 litres of fluid, and should last around 9,000 miles.  

If you’re worried about your AdBlue levels and aren’t sure what to do, you can easily take your car for an interim service where they will perform all the basic checks and look after your levels. 

Can you refill AdBlue yourself? 

There are two main systems on cars fitted with AdBlue, user-fillable and service-filled. The former, as we’ve touched on already has a separate filler cap for AdBlue which is usually located next to the main diesel filler, in the boot or even in the car bonnet. 

Some other cars have a hidden filling system that is usually under the boot carpet, these systems are designed to be filled by the service centre at your regular service intervals.  

How do you know if your car needs AdBlue? 

If you’re unsure of which system your car uses, or are borrowing or renting a car and the owner didn’t tell you whether to add in AdBlue, there are a number of key things to look out for: 

  • An AdBlue cap near the fuel filler 
  • The car model has ‘blue’ in the name, such as Mercedes BlueTEC, or VolksWagen’s BlueMotion 
  • The handbook notes the AdBlue filler location 
  • The onboard computer will advise when the AdBlue tank needs to be refilled 

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