What is AdBlue and what does it do? 'Diesel exhaust fluid' explained | Carbuyer (2024)

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In a move designed to decrease the amount of nitrogen oxides emitted from their exhausts, modern diesel cars now require diesel exhaust fluid (DEF), commonly marketed as AdBlue. AdBlue has been used in exhaust systems of new diesel cars since the introduction of the Euro 6 emissions regulations in 2016 and will also be used in Euro 7 diesel cars from 2025. This guide explains how AdBlue works, what it’s used for, what happens if your car runs out of AdBlue and where to buy it if you need it.

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AdBlue is essentially a mix of urea and deionised water. When your diesel car goes in for a service, your AdBlue should be topped up if necessary to make sure the exhaust gas cleaning system is working correctly and keeping emissions down.

You can also check AdBlue levels yourself and should know how to top it up in case you run low between services. Modern diesel cars also have a dashboard warning light to tell you if you’re running low, just like when you run out of washer fluid or run low on oil. You can buy AdBlue from major retailers such as Halfords.

What does AdBlue do to my car and how does it work?

All new cars must conform to strict emissions standards put in place to minimise their environmental impact. AdBlue is needed to reduce the harmful emissions in the exhaust gas of your diesel car, helping it meet these strict standards.

Although diesels typically emit less CO2 than petrol engines, they tend to produce higher nitrogen-oxide emissions, which have become the source of much concern in recent years because of the damaging effects they can have on public health.

AdBlue is used in the selective catalytic reduction (SCR) system of your diesel car. The SCR process involves injecting precise amounts of AdBlue solution into the exhaust system and neutralising the harmful emissions in the exhaust gases by way of a chemical reaction. When the solution combines with exhaust emissions, it breaks down the harmful mono-nitrogen oxides present in diesel exhaust gases. This technology has been used in buses and heavy lorries for a long time, so its effectiveness has been proven and its reliability is better than ever.

What is AdBlue made of?

AdBlue is a non-toxic liquid that’s colourless in appearance and is essentially a solution of water and urea – a substance found in urine. However, in AdBlue, the urea is exceptionally pure and is of a higher grade than that used in cosmetics, glue or fertilisers. Similarly, the water is demineralised, which is far cleaner than water from the tap.

Can I refill AdBlue myself and how do I do it?

Yes, you can refill AdBlue yourself. Your AdBlue will be topped up every service and your dealer will happily refill it at other times when required, but this is rarely the cheapest option.

On several mainstream diesel models, the AdBlue filler is located behind the car’s fuel filler cap. It’s usually smaller than the main fuel filler, and will feature a blue cap and markings confirming it should only be used for AdBlue. While AdBlue may look like water, do not refill the tank with ordinary tap water – you run the risk of causing damage to the SCR system.

If you’re unsure of how to top up your car’s AdBlue, you should refer to the owner’s manual for instructions on how to access the tank - it shouldn’t be difficult. It’s also a good idea to ask the seller to show you how to refill the AdBlue during the handover of a new car.

Where can I buy AdBlue?

If you choose to top up your AdBlue yourself, you can purchase it in bottles at fuel stations or you can order it online. It’s worth pointing out that AdBlue has an expiry date and shouldn’t be used beyond this point as it may damage the SCR system, so don’t stock up on huge quantities.

• Buy AdBlue refills from Amazon here.

When buying AdBlue, you should check it meets the correct specification, so look for the ISO 22241 number on the packaging. This may also appear as ISO-22241-1, ISO-22241-2, ISO-22241-3. This will ensure the AdBlue doesn’t damage your car’s SCR catalyst – a costly repair. Assuming your AdBlue meets these specifications, one brand of AdBlue should be pretty much the same as another, in the same way that diesel fuel is fundamentally the samefrom one retailer to another.

Where can I find an AdBlue pump?

Another option for topping up your AdBlue tank is to use an AdBlue pump. These can be found at most big filling stations in the HGV lanes. Some of the pumps feature a specific fuelling nozzle for HGVs and a different one for cars.

An AdBlue pump is usually used by truckers and is often far cheaper and less messy than trying to top-up your tank from a plastic bottle. The filling stations with AdBlue pumps were originally largely restricted to key routes and motorways but more have been added in recent years.

I have an AdBlue warning light? What should I do?

All diesel cars that use AdBlue will give you plenty of warning if you're running low, so don’t panic if your light has just turned on. You’ll usually be alerted with a dashboard warning at around 1,500 miles from running out, along with an amber warning light. This warning will remain on every time you restart your car until the AdBlue levels have been topped up to the desired level.

AdBlue warning display

Can I drive without AdBlue?

Ignoring the AdBlue warning light on your dashboard is not advised under any circ*mstances. If you run out of AdBlue while driving, you will still be able to drive, but your car’s performance will likely be affected as it tries to reduce its emissions output by going into ‘limp mode.’ This will reduce the speed at which you can drive and sometimes turn off your vehicle’s stereo or air conditioning to preserve power.

Once you’ve stopped, the majority of modern cars cannot be restarted while the AdBlue tank is completely empty. Fortunately, this situation is easily avoidable, as AdBlue refills are straightforward and usually relatively cheap if you shop around and do them yourself.

Does AdBlue affect fuel consumption?

Manufacturers have yet to release any data to suggest that AdBlue has an adverse effect on fuel consumption. Economy figures for a new diesel car on sale in the UK will factor in any effect from the use of AdBlue in any case.

Developments in engine technology, changes to the way economy figures are calculated and a range of other variables means it’s essentially impossible to find differences in fuel consumption between new and older cars and attribute them solely to the use of AdBlue.

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