What is diesel exhaust fluid (DEF)?

Diesel exhaust fluid (DEF) is used in diesel vehicles to help to reduce levels of dangerous exhaust emissions released by these vehicles.

It works by reducing nitrogen oxide (NOx) particles, which when released into the atmosphere can be harmful to human health.

DEF became compulsory in 2015 when the EU introduced the Euro 6 emissions standard.


What is AdBlue?

AdBlue is a brand name registered by the German Association of the Automotive Industry,

AdBlue is often used interchangeably with 'diesel exhaust fluid' (DEF) as it is the most recognised form of diesel exhaust fluid.

AdBlue and other DEFs can reduce emissions of NOx by up to 90%.

What is AdBlue made of?

AdBlue /a diesel exhaust fluid (DEF) is a colourless fluid, composed of de-ionised water (67.5%) and urea (32.5%),


What does AdBlue do: How does AdBlue work?

AdBlue fluid is required for diesel engines, for selective catalytic reduction (SCR). helping to lower nitrogen oxide emissions.


How does AdBlue reduce exhaust emissions?

The AdBlue fluid enters a vehicle's exhaust system, where the aqueous urea is converted into ammonia and carbon dioxide.

This ammonia (NH3) reduces the amount of NOx particles by converting them into harmless water (H20) and nitrogen (N2), which are themselves released into the atmosphere as exhaust gases through the exhaust system.


Adding DEF to your vehicle

Diesel Exhaust Fluid - or AdBlue - must be added to a separate AdBlue tank.

The AdBlue cap is usually adjacent to the diesel cap, but is sometimes located under the bonnet.

Your vehicle's handbook will cover any fitted AdBlue technology, including how to add DEF safely.



Care must be taken not to contaminate the AdBlue tank.

Accidentally adding diesel fuel or engine oil to the AdBlue tank, for example, will seriously impact how well the SCR system works.

Equally, accidentally contaminating a diesel tank with AdBlue will cause serious problems.

If you do accidentally mix the two, DO NOT start the car engine but instead call your vehicle breakdown recovery provider.


What will happen if I get AdBlue in my car’s diesel tank?

Getting AdBlue in your car's diesel fuel tank is likely to result in a blocked filter which will mean no fuel enters the engine - and the vehicle will not proceed.


AdBlue running low

Modern diesel vehicles display an AdBlue warning light on the dashboard, to alert the driver when AdBlue levels are getting low and need to be topped up.


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

Most vehicles allow for 1,000 – 1,500 miles once the AdBlue warning light comes on, so there’s no need to panic.

That said, you should avoid delaying the top up of your vehicle’s AdBlue.


Running out of AdBlue/DEF

Many modern diesel vehicles will not start next time if the DEF has run out.

With some vehicles, if your diesel engine runs out of AdBlue whilst driving, they will enter ‘limp mode’ to enable you to get home or somewhere to top up. That is, the engine power and performance will be reduced to limit emission levels.


AdBlue and MOTS

Your vehicle’s emissions are checked as part of the MOT so it is important to ensure you’re your AdBlue is topped up .– If your car fails its exhaust emissions test during an MOT check, you will fail your MOT.


Clearing the Adblue Warning Light

Once you have refilled your Adblue, the dashboard warning light should automatically switch off..


How long will the AdBlue in my car last?

Only a small amount of AdBlue is used per journey, so the need to refill AdBlue is required only periodically.

A full tank of AdBlue is likely to last for several thousand miles of driving.

It’s worth noting that your driving style can have an impact on how often you will be required to refill your cars AdBlue; the more economical your driving style, the less AdBlue you’ll use (and careful driving will help to decrease fuel consumption!)


Where can you get AdBlue from?

AdBlue diesel exhaust fluid can be added at most fuel stations through a DEF/AdBlue pump nozzle, just like diesel or petrol.

Alternatively, a bottle of AdBlue can be purchased and added by the owner.

AdBlue can be purchased from various outlets including automotive retailers, garages, online retailers and some supermarkets and motorway service stations.

Once again, the car operator’s manual should be consulted thoroughly in relation to using AdBlue safely, including any minimum/maximum AdBlue refill quantity.


Which vehicles use AdBlue technology?

In order to meet European emissions regulations (Euro 6), most new diesel vehicles must be fitted with AdBlue/DEF technology.

Alongside manufacturers of lorries, coaches, buses and agricultural vehicles, many diesel passenger cars are also fitted with SCR systems.


Which cars use AdBlue?

Engines over 1.6-litres in capacity will in most cases feature AdBlue technology.

Manufacturers include Volkswagen, Audi, Mercedes, Peugeot, Citroen and Jaguar.

Without AdBlue diesel technology, many diesel cars would not meet strict new emissions targets.

Most diesel-powered cars built since 2006 feature AdBlue technology.


Is all AdBlue the same?

The price of AdBlue can vary significantly so it’s no surprise many people wonder of all AdBlue is the same.

To ensure you’re using the correct diesel exhaust fluid:

  • Only buy AdBlue from reputable retailers
  • Check for “AdBlue according ISO 22241” on the container or dispensing system. This may also appear as ISO-22241-1, ISO-22241-2 or ISO-22241-3.
  • Watch out for similar brand names that could be inferior in quality
  • Be wary of any AdBlue sold at extremely low prices


It is always best to check with a professional before purchasing any additive.


Is AdBlue dangerous?

The urea and de-ionised water comprising AdBlue are completely harmless to humans.

That said, AdBlue can cause irritation to the skin, so it is advisable to wear gloves when filling your diesel cars AdBlue tank and to wash your hands after.


Can AdBlue damage my vehicle paintwork?

AdBlue fluid can corrode bodywork if spilled, so care must be taken when transporting and filling.


Installing an AdBlue system in an older diesel car

It is uncommon for an older diesel car to be retrofitted with an AdBlue/DEF system, due to the cost, time and complexity of the project. Retro-fit systems do exist however, including the BlueFit from Amminex.

As with any type of car modification, the insurer should be informed that an AdBlue def system has been added. The premium may be impacted as a result of such a modification.


Automotive urea

For the curious, the urea used in DEF is completely man-made, created by introducing heat to synthetic ammonia and carbon dioxide.

The manufacturing of DEF ensures that the urea is pure and free of contamination, and the right ratio to the deionised water. 


Is your diesel car ULEZ compliant?

Older diesel cars that do not have DEF systems are generally not ULEZ compliant. Find out if your diesel vehicle is required to pay the ULEZ charge.


Car Maintenance

For more car maintenance tips visit Car Servicing and Repairs | startrescue.co.uk