Search our range of cars

Avoid the AdBlue breakdown blues

By Benchmark

It reduces harmful emissions but if you run out, your car won’t start. 

Many new diesel cars use a fluid called AdBlue, also known as Diesel Exhaust Fluid or DEF, to help reduce harmful exhaust emissions. If your car uses this technology there’ll be a smaller blue filler cap for the AdBlue tank next to the main diesel filler. It may also be found in the boot or under the bonnet.

It's important to understand that AdBlue's an exhaust additive, kept in its own special tank and injected automatically in small quantities into the flow of exhaust gas. AdBlue is not a fuel additive and you will cause expensive damage if you add it to the diesel tank by mistake.

It may surprise you to learn that your car won’t start if the AdBlue runs out. And in fact, this has certainly come as a shock to many of our members, especially, it seems, those driving company or rental cars.


With diesel accounting for 47% of new car sales, the number of cars relying on AdBlue technology is going to increase rapidly over the next few years. So, how do you avoid the AdBlue breakdown blues?

Most diesel drivers will need to top up the AdBlue at least once between services, so it’s important to keep an eye on dashboard warnings, particularly if you’re a high mileage motorist.

What is AdBlue?

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

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

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

What happens if I put AdBlue in the diesel?

AdBlue isn’t a fuel additive and 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.

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.

How does AdBlue work?

Tiny amounts of AdBlue are injected into the flow of exhaust gasses. At high temperatures AdBlue turns to ammonia and carbon dioxide. Inside the SCR catalyst, harmful nitrogen oxide in the exhaust reacts with the ammonia and is transformed to harmless nitrogen and water.

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

Does my car 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.

Some older cars or vehicles from other manufacturers use AdBlue too so other ways to check include:

• The vehicle handbook

• An AdBlue filler cap either next to the fuel filler, in the boot or under the bonnet

• ‘Blue’ or ‘SCR’ in the model name

If you’re still not sure, ask a dealer.

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’s running low – usually a text warning on the dashboard with around 1500 miles to go followed by an amber warning light.

How do I refill the AdBlue tank?

It’s easy to top up with AdBlue if you need to between services.

AdBlue is widely available in 1.5 litre, 5 and 10 litre containers from car accessory shops and filling stations. The smaller packs have a specially designed neck which allows you to top up the tank without a funnel but without risk of spillage, so why not carry a small container of AdBlue so you’re ready to top-up when necessary?

• 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/design of the AdBlue pack you’ve got.

• Wash your hands and rinse any spills from the bodywork – AdBlue’s non-toxic but can cause irritation to your skin and eyes and will damage the paintwork.

How often will I have to refill the AdBlue tank?

The rate at which cars use AdBlue varies a lot depending on the engine and how economically you drive.

Typical consumption seems to be around a litre of AdBlue every 600 miles but could be as high as a litre every 350 miles. The size of the tank varies too so the range between refills could be 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.

How does AdBlue reduce harmful exhaust emissions?

Vehicles have to meet strict exhaust emissions limits and the latest standard, Euro 6, is very challenging on Nitrogen Oxide (NOx) limits. NOx gases cause respiratory problems and contribute to the formation of particulate matter, smog, acid rain and ground level ozone.

Euro 6 applied to new models from September 2014 and to all new diesel cars from September 2015.

Most cars can only meet the Euro 6 standard when fitted with emissions technology known as Selective Catalytic Reduction. This ‘SCR’ uses a fluid called AdBlue, injected in small amounts into the car’s exhaust, to break down harmful nitrogen oxide emissions into harmless nitrogen and water vapour.