Petrol and diesel combustion engines inevitably produce emissions as part of their normal operation. The Euro 6 standards were introduced within the EU to try and ensure vehicle manufacturers keep harmful emissions below specific limits. The rules cover nitrogen oxide (NOx), carbon monoxide (CO), hydrocarbons (THC and NMHC), and particulate matter (PM). The last of which is the soot you can see emanating from the rear of diesel vehicles.

Limits are different for petrol and diesel engines:

  • Euro 6 Diesel Emissions Standards (grammes per kilometre): 0.50 CO, 0.080 NOx, 0.005 PM
  • Euro 6 Petrol Emissions Standards (grammes per kilometre): 1.0 CO, 0.060 NOx, 0,005 PM

The AdBlue tank next to the Diesel tank

Many new diesel cars use a fluid called AdBlue® which 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.

What is AdBlue®?

While there are different names for AdBlue®, such as Bluedef, BlueTec and so forth, all mixes should contain the same solution of 32.5% urea and 67.5% de-ionised water. The German Association of the Automotive Industry’s AdBlue® marque can be confidently relied upon to meet these requirements, while others may not.

You should not use water instead of AdBlue®. The VDA notes that vehicles should not use urea solutions not denoted by the ‘AdBlue’ trademark. While inexpensive, these can be dangerous, damage the engine, affect steering, and harm the SCR catalytic converter, so it’s always best to stick with legitimate, branded 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).

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.

What Does AdBlue® Do?

the AdBlue tank closed up

AdBlue® is considered to reduce NOx emissions by over 90%, making it a crucial tool for improving air quality and combating environmental issues. It is injected into the exhaust, where the aqueous urea solution evaporates and splits into ammonia and carbon dioxide. In turn, the ammonia turns the harmful nitrogen oxides into water and nitrogen, both of which are harmless. This process is managed by a Selective Catalytic Reduction (SCR) system.

AdBlue® has no direct effect on the functioning of the engine. It helps reducing the polluting emissions to meet the standards of the European norms. However, in case of non-refill of the AdBlue® tank, the performance of the engine can be limited, or the vehicle stopped. 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 running low. You'll usually see a text warning on the dashboard at around 1500 miles to go followed by an amber warning light.

The use of SCR technology and AdBlue® enables the optimisation of combustion, which can lead to a reduction of consumption for some vehicles. This consumption reduction is already integrated into the figures claimed by car manufacturers.

How Can I Top Up my AdBlue®?

AdBlue being refilled in a truck.

It’s fairly straightforward to top up the AdBlue® in your vehicle, either from a container of an appropriate size for easy refill, from a pump at a service station, using an AdBlue® pump (which are available as hand operated, battery powered, or mains powered), or if you’re not confident topping up your own AdBlue® Halfords offer a service.

The location of the AdBlue® tank filler neck depends on the model of your vehicle. On certain models, the AdBlue® tank filler neck is next to the Diesel filler neck. On others, the AdBlue® filler neck is inside the boot, and on other vehicles, the filler neck is under the bonnet. To get this information, contact your dealership, or see the owner manual of your vehicle.

AdBlue® is sold by petrol stations, garages, motor accessory shops, online retailers and even some supermarkets. 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.

You can find filling stations which have AdBlue® pumps. It is important to always ask if the pumps are designed to refill the tank of a passenger car. That is not always the case. It is likely that only heavy-duty trucks can refill at the pump. Indeed, heavy-duty trucks must often refill AdBlue®, sometimes weekly, so the station pumps are specifically designed for their use (particularly the flow speed, adapted to the size of their tanks which is much bigger than that of passenger cars).

According to the AA, 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.