CLM’s Guide to AdBlue

With most new diesel cars now featuring AdBlue, we’ve produced this handy guide
to answer the most common questions that we get about the technology.

What is AdBlue?

AdBlue is actually a brand name owned by the German car manufacturers association, but it has become the commonly used term to describe the urea-based liquid which many modern diesel vehicles use to reduce their tailpipe emissions of Nitrogen Oxides.

Nitrogen Oxides (NOx – most commonly Nitrogen Oxide and Nitrogen Dioxide) are blamed for causing respiratory conditions in those living in urban environments and are the chemicals behind city smog. The latest Euro VI emissions standards demanded a 67% drop in NOx exhaust emissions of diesel cars in order to combat these problems. The widescale adoption of Adblue technologies by motor manufacturers is one of the key ways to achieve this.

AdBlue BP station

What is AdBlue made of and how does it work?

Despite the urban myths, the urea used in AdBlue is entirely synthetic and is not refined animal urine. The urea is mixed with de-ionised water at a ratio of roughly one part urea to two parts water, to produce the compound which is injected in precise quantities into the exhaust flow of the engine.

When this happens the AdBlue liquid releases ammonia, which helps to convert the Nitrogen Oxides into water vapour and Nitrogen through a process known as selective catalytic reduction. As almost 80% of our atmosphere is Nitrogen (and generally between 1-4% water vapour) these emissions are entirely harmless.

Does my car use AdBlue?

If your diesel car is new then there’s a very good chance that it uses AdBlue technology.

The best place to look is in the handbook, but you may also spot tell-tale signs such as a small additional filler cap next to the main diesel filler, or the word ‘Blue’ somewhere in the model’s name or specification.

In some cars the filler might be found in the boot or the engine bay, but this is becoming less common. It’s important to remember that AdBlue isn’t a fuel additive and it should never be mixed with the car’s fuel as this can cause severe damage to the engine. That’s why it is always stored in a separate tank.

How often does the AdBlue tank need topping up?

When AdBlue was first introduced into cars it was assumed that the dealer would fill up the tank at each service. However, as service intervals have extended, it’s quite likely that at some point you’ll need to top up the tank yourself. The time between top-ups obviously depends on the size of the tank and how many miles you cover, but it’s also impacted by driving style. Just like with fuel consumption, rapid acceleration, interspersed with sharp braking, will use more AdBlue than a smoother driving style.

Your car will let you know as you use the AdBlue reserves in the tank, and will prompt you to fill up when the liquid gets below a certain level.

What happens if I run out of AdBlue?

Given its ready availability and the fact that your car will warn you (and become increasingly insistent as levels get very low), there’s not really an excuse for running out of AdBlue. If you are driving and the reserves are completely depleted the performance of the car will be reduced, as the engine management system attempts to remain within legal emission limits.

If you do run out, the engine won’t be damaged but it’s essential that you find the nearest service station to fill up as soon as possible. It is worth noting that some engines won’t restart if they have run out of AdBlue completely and may require a trip to the dealer to get them reset. To prevent this, it’s worth carrying an emergency supply of AdBlue and topping up the tank with the engine still running if you do run out.

Adblue and Diesel

What happens if the AdBlue and Diesel get mixed together?

If you somehow get diesel in the AdBlue tank do not start the engine. Doing so is likely to cause damage to the selective catalytic reduction and AdBlue injection systems and they may then have to be replaced at significant cost. The best advice is to call your breakdown service to have the AdBlue tank drained.

Likewise, if you get AdBlue in the diesel tank, do not start the engine as this is likely to cause catastrophic damage to the entire fuel system. Your diesel tank will need to be drained and thoroughly flushed through.

Other AdBlue dos and don’ts

  • Only fill the AdBlue tank with the diesel filler cap securely in place and vice versa – it only takes a few drops in the wrong tank to cause damage.

  • AdBlue will freeze at around -12C, however this won’t damage the fluid once it thaws out.

  • AdBlue can, however, be damaged through prolonged exposure to temperatures over 30C and can also be degraded by sunlight.

  • Despite AdBlue being mainly water do not be tempted to water down AdBlue as this will cause serious damage to the system (AdBlue is manufactured with highly purified de-ionised water).

  • Don’t be tempted to fill your AdBlue tank with a cheaper alternative labelled ‘Urea solution’ or something similar. The precise combination of urea and de-ionised water is required. Look for the standard ISO 22241 to ensure that the product is compatible with AdBlue technology.

  • If you think your AdBlue may have been contaminated by any other liquid or even dirt or dust do not use it.

  • AdBlue should ideally be used at the time of opening the container as it does degrade quite quickly when exposed to the air.