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.