What is AdBlue?

What is AdBlue and how does it work?

AdBlue reduces harmful emissions – but if you run out, your car won’t start

Many new diesel cars use a fluid called AdBlue. If your car uses AdBlue, you'll probably need to top it up at least once between services.

Find out what it does, how it works and why it’s important to keep an eye on dashboard warnings – particularly if you’re a high-mileage motorist.

Adblue filler cap

What is AdBlue?

AdBlue is an exhaust fluid, not a fuel additive. It's stored in a separate reservoir and is topped up via a (usually) blue filler cap located either next to your fuel filler, in the boot or under the bonnet.

It's a trade name registered by the German car manufacturers association, but is the most recognised form of Diesel Exhaust Fluid (DEF).

What is AdBlue made of?

AdBlue is a colourless, non-toxic mixture of urea and de-ionised water. It’s not actually blue at all.

Lots of people think AdBlue is made of pig urine – it’s not. The urea used in AdBlue is a high-purity man-made solution – pig urine wouldn’t be pure or sterile enough for a commercial product.

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How does AdBlue work?

Tiny amounts of AdBlue are injected into the flow of exhaust gases. At high temperatures AdBlue turns to ammonia and carbon dioxide.

Inside the SCR (Selective Catalytic Reduction) catalyst, harmful nitrogen oxide in the exhaust reacts with the ammonia and is transformed in\to harmless nitrogen and water.

Similar technology has been used effectively for years in buses and heavy lorries.

Need breakdown cover? Read our guide about how it works:

What happens if I run out of AdBlue?

If you run out of AdBlue while you’re driving, then the engine’s power and performance will be reduced to limit its emissions. Once you’ve stopped, you won’t be able to restart the engine if the AdBlue tank’s empty.

The car will give you plenty of warning that the AdBlue tank is running low. You'll usually see a text warning on the dashboard when there's around 1500 miles' worth of AdBlue left.

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Does my car use AdBlue?

Some older cars or vehicles from other manufacturers use AdBlue too. So if you’re not sure if your car uses it, you can check:

  • The vehicle handbook.
  • See if there’s an AdBlue filler cap either next to the fuel filler, in the boot or under the bonnet.
  • If the model name contains ‘Blue’ or ‘SCR’.
  • If you’re still not sure, ask a dealer.

Where can I buy AdBlue?

AdBlue is sold by petrol stations, garages, motor accessory shops, online retailers and even some supermarkets. If you’re in any doubt that you’re buying the right stuff, contact your dealer.

It’s widely available in 1.5, 5 and 10-litre containers. The smaller packs have a specially-designed neck that lets you top up the tank without risk of spillage even if you don't have a funnel.

With easy-fill bottles available, it's a good idea to carry a small container of AdBlue so you’re ready to top up when necessary.

How long does AdBlue last?

The rate at which you use AdBlue depends on your engine and how economically you drive.

  • Typical consumption is around a litre of AdBlue every 600 miles.
  • But it could be as high as a litre every 350 miles.
  • Tank size varies too, so you could need to refill somewhere between 3,000 and 12,000 miles depending on the car and your driving style.

This means that most drivers will have to top-up their AdBlue reservoir at least once between normal service visits to the dealer.

Is it safe to drive with the AdBlue warning light on?

It’s not a major safety issue, but if you don’t top up, you’ll eventually run out of AdBlue and you won’t be able to start the car.

When the warning light first comes on you’ll have plenty of time to refill – so don’t panic. Just make sure you don’t put off topping up, especially if you’re about to book an MOT test.

How to reset the AdBlue warning light

There’s no manual way of resetting your AdBlue warning light – the only thing you can do is top up. Once you’ve refilled to the minimum level, the warning light will normally disappear (though you may need to go for a short drive for the refill to register).

Where is the AdBlue tank?

The AdBlue tank is usually located under the boot floor, where the spare wheel might have been in the past. But it's more important to know where the filler for it is located.

There'll usually be a small blue filler cap next to the main diesel filler. If it's not there, then check in the boot or under the bonnet.

What happens if I put AdBlue in the diesel tank?

Putting it directly into your fuel tank could cause expensive damage to your fuel tank, pump and injection system. If you start the engine it's likely to be an expensive write-off, so don’t touch the ignition.

You’ll need to get the tank drained and the contents disposed of safely before you can refill it. Call us, and we’ll sort the problem out.

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What happens if I put diesel in the AdBlue tank?

Don’t turn on the ignition or start the engine and call us for help.

If you do start your engine, then the damage could mean that you have to get the entire Selective Catalytic Reduction and AdBlue injection system replaced.

Find out more:

How do I refill the AdBlue tank?

  • Follow any instructions given in your handbook or on the pack.
  • Your handbook will tell you how much AdBlue the tank holds.
  • 5 litres should be enough to make sure your car will restart if you’ve run out completely.
  • You may need a funnel depending on where the AdBlue filler cap is located (next to the fuel filler, in the boot, or under the bonnet) and the size or design of the AdBlue pack you’ve got.
  • Wash your hands and rinse any spills from the bodywork – AdBlue is non-toxic but can cause irritation to your skin and eyes and may damage the paintwork.

Can you overfill the AdBlue tank?

 It’s possible to overfill the AdBlue tank on some cars, but it’s more likely to make a mess as it spills out of the filler cap, than to cause any damage. Pay attention to the size of your AdBlue tank and how many litres it can hold. If it’s only 8 litres then make sure to add around 7 litres, just to be safe.

How much does AdBlue cost?

AdBlue roughly costs around £8 for a 5L container, but prices can vary quite wildly if you’re looking to purchase some online. Make sure to check several sources before making your mind up.

Is all AdBlue the same?

You might see AdBlue with different names depending on the brand that’s making it, but the basic mixture doesn’t change. The solution should always be 32.5% urea and 67.5% de-ionised water.



Which cars can use AdBlue?

Not all car manufacturers use AdBlue technology. But chances are if you’ve got a diesel from Jaguar, Land Rover or any French or German manufacturer, and it was registered new after September 2015, it’ll use AdBlue.

How does AdBlue reduce exhaust emissions?

AdBlue is added to your exhaust and mixes with the fumes your car produces. It reacts with nitrogen oxide gas (NOx gas) created by your engine and breaks it down into harmless nitrogen and water vapour.

NOx gases cause respiratory problems and contribute to the formation of particulate matter, smog, acid rain and ground level ozone.

Vehicles have to meet strict exhaust emissions limits and the latest standard, Euro 6, is very challenging on Nitrogen Oxide (NOx) limits.

Most cars can only meet the Euro 6 standard when fitted with emissions technology known as Selective Catalytic Reduction (SCR). This technology uses AdBlue to break down and reduce harmful emissions.

Is AdBlue toxic?

While AdBlue isn’t classified as toxic, it could still cause environmental damage if allowed to run into a source of water, and it can also be an irritant if it gets on your skin or in your eyes.

Can I refill AdBlue myself?

Yes, you can refill AdBlue yourself. Typically you’ll find an easily accessible AdBlue filler cap right next to the diesel filler cap. Simply unscrew the cap and pour in the AdBlue solution.

Is it ok to drive without AdBlue?

Once you’ve completely run out of AdBlue you won’t be able to start your car. You’ll get plenty of warnings from your dashboard when your AdBlue is getting low though, so make sure to get on top of it as soon as possible.

Driving with little or no AdBlue will reduce the performance of your engine, as it will compensate in order to reduce emissions.

How long does a full tank of AdBlue last?

Most tanks of AdBlue will get you through around 6,000 to 7,000 miles, but this can vary depending on your car and the size of the tank.

How do I start my car after running out of AdBlue?

If you’ve completely run out of AdBlue, your car won’t start again until you’ve refilled typically around 3-5 litres. Once your car starts, the AdBlue warning light may take some time to catch up. Don’t worry, it’ll soon turn off once you’ve driven for a while.

Published: 1 May 2017 | Updated: 19 May 2023 | Author: The AA

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