How does AdBlue work and does your car use it?

You may have seen bottles of AdBlue at petrol stations or mention of it in new car brochures, but what does it do and does your car need it? 

What is AdBlue?

You may have seen bottles of AdBlue for sale at petrol stations or online, seen it mentioned in car brochures or on the internet, but unless your car needs it there's a good chance you have no idea what it is or what it does.

Put simply, AdBlue is an additive that is used to treat the exhaust gases coming out of cars and reduce nitrogen-oxides - the harmful NOx that have become a major talking point in recent years and one of the causes of smog in urban areas.

Many Euro6-compliant vehicles have an AdBlue system to help them achieve the required targets, so it is not something you need to worry about if you have an older diesel vehicle. AdBlue is a non-toxic, colourless solution of urea and water-based fluid and it's also worth mentioning that AdBlue is a trading name - technically it should be called diesel exhaust fluid, but most retailers simply use the trade name.

How does it work?

The science bit sees miniscule amounts of Adblue added to the exhaust gases coming from the engine, and inside the high temperatures of the Selective Catalytic Reduction (SCR) catalyst the AdBlue turns into ammonia and carbon dioxide. The ammonia reacts with the nitrogen oxides in the gases and are turned into nitrogen and water - neither of which are harmful.

None of this produces any noticeable effect on how the car drives or how you operate it. It operates automatically when you drive your car, so there's no need for any additional input from you, save for when the tank requires refilling.

Which cars use AdBlue?

Only recently-built, diesel cars will have AdBlue systems fitted and many people are unaware the system is installed until a warning message appears.

Many German cars are fitted with the system, particularly those with larger engines built after 2015. Some models from Audi, BMW, Mercedes-Benz, Citroen, Peugeot, DS, Mazda, Renault, Jaguar, Land Rover, Volkswagen, SEAT and Skoda require AdBlue, but your handbook will confirm if your specific model uses the system.

Depending on the model, refilling the tank can either be performed by you or in some instances only by a dealer. Again, checking the vehicle handbook will give you the necessary information, although if you've noticed an additional blue cap next to the regular diesel filler cap then it is safe to assume you can fill it yourself.

When do I need to refill my AdBlue?

Because AdBlue is a relatively recent addition to new cars, the information display on your dashboard will let you know if your AdBlue is running low. As it is a critical system to engine performance it should give you plenty of notice too - typically at least 1000 miles before the tank is dry.

It may be tempting to run the tank dry as you might do with a fuel tank, but in the same way that modern, sophisticated diesel engines don't appreciate being run until they are empty, a bone-dry AdBlue tank will have consequences too.

Because the system has a significant effect on the emissions performance of the engine, they are designed so that they cannot be driven in a normal fashion if there is no AdBlue available. That means most cars will go into limp mode in this instance.

If you've not experienced limp mode, this is when the car's Engine Control Unit (ECU) reduces engine power and can even lock out some higher gears in automatic transmissions, so you have to literally crawl along at low speeds. If the car is dry when you try to start it, it may not even fire up at all until the tank is replenished.

It's also worth noting that, unlike a petrol tank, you can't just stick a fiver's worth of AdBlue in to get going. Most systems will require a significant top-up before it will operate normally once more, so it's worth buying enough to fill the tank.

Can you refill AdBlue yourself?

It depends entirely on your particular model of car as to whether you can refill it yourself. Some vehicles have the refiller located underneath the carpet, under the bonnet or other places that are intended to discourage you.

However, those that are designed to be topped up at home put the filler next to the fuel filler cap, so it is an easy job.

As always, check the vehicle handbook for the correct information, particularly if you are filling at home. Although AdBlue isn't a fuel and so isn't flammable, it is worth treating it with care.

Mop up any spillages, avoid contact with clothing and wear gloves, or wash your hands thoroughly afterwards. AdBlue only lasts for 12 months, so don't be tempted to stockpile, and ideally it should be kept in storage above -10C and below +30C.

How much AdBlue will my car use?

AdBlue consumption varies from vehicle to vehicle (for example a Volkswagen Passat gets through about 1.5 litres every 620 miles). But - like fuel consumption - the faster you drive, the more AdBlue you'll burn through.

Most AdBlue tanks hold around 10 litres or more, so most average drivers will find that AdBlue - like screenwash - will be topped up at the annual service, but unless you drive a low mileage it is likely you will need at least one top up between services.

How much does AdBlue cost?

Inevitably it is cheaper to buy your AdBlue from an online retailer or car accessory store than from a dealership, so shop around if you have the choice.

We found 10 litres for just over £10* and 5 litre bottles for around £7*, so don't pay much more than a pound per litre. Peugeot will refill your AdBlue for a fixed price of £9.99, and Halfords* charge £19.99 if you don't want to tackle it yourself.