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Tips and advice

What is AdBlue? Diesel exhaust fluid explained

Most modern diesel cars require AdBlue – here’s everything you need to know

adblue bottle

Since late 2015, all new diesel cars have been required to use AdBlue, a fluid that reduces the amount of harmful nitrous oxides emitted in the exhaust gas. Also known as diesel exhaust fluid (DEF), AdBlue is colourless, non-toxic, and will need to be topped up occasionally.

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. Your service centre will top up AdBlue levels for you, otherwise it’s fairly straightforward to do it yourself.

Car dashboard symbols and meaningsCar dashboard warning lights: the complete guide

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.

What does AdBlue do?

While diesels typically emit less CO2 than petrol engines, they tend to produce higher nitrogen-oxide emissions, which have been proven to negatively impact public health. AdBlue is used to reduce these harmful diesel emissions and allow them to comply with the latest European emission standards.

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Since September 2015, all new diesel cars have conformed to Euro 6 regulations, with the stricter Euro 7 standard coming into force in 2025. If you own a diesel car built from September 2015 onwards, it will use AdBlue in order to meet these standards.

macan diesel badge

AdBlue is used in the selective catalytic reduction (SCR) system of your diesel car. An injector sprays small amounts of AdBlue into the exhaust system, neutralising harmful emissions through a chemical reaction. 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.

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Every now and then, the AdBlue level will need to be topped up so the SCR system can continue to function correctly. This is usually performed by a service centre during your car’s service, but it’s also possible to do it yourself.

I have an AdBlue warning light, what should I do?

The warning light will come on if the system is running low on AdBlue, but it will give you plenty of warning – you’ll usually be alerted with an AdBlue warning when it’s 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.

AdBlue warning

AdBlue warning display

You shouldn’t wait too long to top up the system as driving with a completely empty AdBlue tank will negatively affect your car’s performance. You can contact your local service centre to top up your AdBlue level for you, or you can purchase AdBlue and fill up the tank yourself.

How do I refill my AdBlue tank myself?

It should be fairly straightforward to refill your car’s AdBlue tank. 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.

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Place a funnel into the filler neck and pour the specified quantity of AdBlue into the tank. The owner’s manual should tell you how much AdBlue to use, as well as how to access the tank if you can’t find the filler neck.

adblue filler

It’s 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. 

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.

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.

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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.

Can I drive without AdBlue?

Ignoring the AdBlue warning light on your dashboard is not advised under any circumstances. If you run out of AdBlue while driving, you will still be able to drive, but your car’s performance is almost certain to be affected.

Your car will try 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. Keep a spare bottle of AdBlue at home to avoid this situation – just make sure it hasn’t expired.

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.

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.

Frequently Asked Questions
If you run out of AdBlue, your car will probably enter ‘limp mode’, reducing engine power. If you turn your car off, chances are, it won’t restart until the AdBlue level is topped up.

We've covered the most fuel-efficient cars on sale – read the top 10 list here...

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Charlie writes and edits news, review and advice articles for Carbuyer, as well as publishing content to its social media platforms. He has also been a regular contributor to its sister titles Auto Express, DrivingElectric and evo. As well as being consumed by everything automotive, Charlie is a speaker of five languages and once lived in Chile, Siberia and the Czech Republic, returning to the UK to write about his life-long passion: cars.

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